"I looked up and saw a monster. The boat that hit us was a big black shape in the sky above me and to me it looked like it had eyes. Then the deck was tilting and I grabbed Helen's hand. 'We have to go now,' I said, and we stepped into the water."

Andrew Sutton's voice drops low as he tells the story. He pushes his glasses up on to his forehead and rubs his hands across his face and takes a deep breath. He has relived that moment from 20 years ago thousands of times, but it still shakes him to talk about it. Like many trauma victims, his memory doesn't vary and the fine details of the accident are as accurate and unchanging as a loop of film that he can replay in his head. But the film sometimes plays when he doesn't want it to. Now 48, he looks younger - tall and rangy, tanned with the broad shoulders and long limbs of a swimmer. He is shaven-headed, casually dressed in creative-industry uniform of jeans, a plain black T-shirt and Birkenstock sandals. But his blue-eyed gaze is steady, and when he speaks he is thoughtful, his words finely measured and considered.

We are sitting in his offices in north London, surrounded by half a dozen computers which scroll through photographs of tropical fish, sharks and stingrays. They are Sutton's own pictures, taken while scuba diving in east Africa and the Pacific. He runs his own photographic and video company now, but at the time of the Marchioness disaster he was a graphic designer. Many of the other passengers had creative jobs, in music or the fashion industry, and some were City workers from the banking sector. At the time, some of the press coverage of the accident was tinged, in Sutton's view, with criticism, as if those young Londoners aboard the boat were from a "fast set" who were somehow killed because they were enjoying themselves too much. It is one of the things he is still angry about. "I wasn't rich, and none of my friends were. I've wanted to tell people that since it happened. But I haven't been able to talk about it publicly until now."

That day, 20 August 1989, the Marchioness had done a normal day's work, taking sightseers up and down the Thames as far as Greenwich to see the landmarks of the historic riverfront: HMS Belfast, St Paul's, the Tower of London and Tower Bridge. In the evening it plied the same route, but this time the running commentary of the skipper's loudspeaker system for the day-trippers was replaced with the steady beat of an on-board disco for a private party. That night, the Marchioness had been hired to celebrate the 26th birthday of Antonio de Vasconcellos, a Cambridge graduate who worked for a merchant bank. Andrew Sutton didn't know Vasconcellos, and found himself at the party because he had been drinking on that Saturday night with his friend, the photographer Chris Garnham. Through work, Garnham knew one of the party organisers, Jonathan Phang, then an up-and-coming star on the modelling and fashion circuit.

Sutton wasn't in the mood for a party, having spent most of that week at home with a bad case of summer flu, but Chris Garnham had suggested that an outing would cheer him up. The two men had worked together on designs for several record sleeves, most recently on "Spend Some Time", a single from the jazz musician Cleveland Watkiss. They had also worked on an album with Mimi Izumi, whose portrait by Garnham is just one of his collection of some of the most prominent artists of the 1980s now held in the National Portrait Gallery.

On that night 20 years ago, Sutton, with his girlfriend Helen, and Garnham and his girlfriend Gillian Mosley, were joined by another close friend, Tony Loh Manyem, a photographic assistant. When they turned up at the Embankment pier shortly before 1am, Jonathan Phang welcomed them on board, dismissing any suggestion that they weren't officially invited. However, feeling like outsiders, Sutton and his group headed for the foredeck of the Marchioness. About half an hour later the boat pushed off, heading east against an incoming tide.

"Tony was a lovely, lovely guy. He was pretty scruffy and had a ponytail, and you wouldn't know it to look at him but he was incredibly fit," Sutton recalls. "He would run to work with a rucksack on his back with a couple of bricks in it, always in training for something. He didn't know the Thames very well and I identified the landmarks as we passed them. The Oxo Tower hadn't been done up back then, and I remember pointing out the shell of the old building and explaining to Tony how it got its name. Chris Garnham brought us a beer and then went inside to dance, and Tony and Helen and I just stayed in the bows watching the cityscape unfold. It's funny, I remember..." Sutton falters for the first time, and clears his throat before continuing. "I remember Tony telling me that he couldn't swim, and I said it didn't matter because we weren't going very far. We were in the middle of London - what could happen?"

According to the dry narrative report of the Chief Inspector of Marine Accidents, the Marchioness was hit by the dredger Bowbelle at 1.46am. With 127 passengers and four crew, the party boat had just cleared Southwark Bridge. The Bowbelle was behind the Marchioness, both boats adjusting their course to stay in the centre of the river to go through Cannon Street Railway Bridge. Somehow they converged, although crew members on the party boat had already seen the dredger coming up behind them and assumed it was going to overtake. The dredger had passed a similar party boat, the Hurlingham, a few hundred yards upstream. At eight and a half knots, the dredger was moving twice as fast as either of the pleasure cruisers. Two crewmen stationed on the bows of the dredger failed to see the Marchioness ahead, and the skipper of the Bowbelle, Douglas Henderson, was not at the helm. He was later criticised for having drunk six pints of beer on the afternoon of the day of the accident, albeit when he was off duty.

As the collision became inevitable, at least one deckhand on the Marchioness shouted a warning to his skipper Stephen Faldo, who applied full throttle to try to escape the bigger boat. Faldo died in the accident, one of the reasons why subsequent inquiries struggled to reach a definitive verdict on what happened. What is certain is that the Bowbelle weighed 1,880 tonnes and was more than 260ft in length. The Marchioness was just 85ft long, and weighed only 46 tonnes. The dredger's iron hull swept the party boat's wooden superstructure aside, crushing the Marchioness with such force that her upper deck separated from the hull. For a short time it was pushed sideways across the dredger's bows and then forced under water like a toy. In less than a minute the Marchioness was completely under water with most of the partygoers trapped in the saloon. Twenty-four of the drowned victims were found below decks, and the remaining 27 recovered from various parts of the Thames over the coming days. They included the birthday boy, Antonio de Vasconcellos, as well as his younger brother Domingo, and the youngest person on board, 19-year-old Francesca Dallaglio, sister of the future England rugby captain, Lawrence.

For Sutton, the subsequent half-hour was deeply traumatic. Because he was at the very front of the Marchioness he escaped physical injury. But once in the water he was aware of the great looming presence of the Bowbelle pushing the party boat aside and then sweeping it beneath the dark surface of the Thames. What he remembers vividly as the "eyes" of the dredger were the "hawse holes" cut in the gunwales for a cable to be passed through.

"I had Helen by the hand. She was screaming and swearing at me, barely able to stay afloat. I tried to swim away from the dredger, dragging her along, but then there was a sensation of something around my legs. I thought I was caught on something and I started to panic. I put my head under water and saw my friend Tony holding on to my thighs, trying to pull himself up. We were face to face and I could see the terror in his eyes. He was being dragged down by the boat, or by the current... I don't know. But I couldn't do anything and I couldn't let go of my girlfriend. And then he was gone and we were alone in the water."

Unsurprisingly, that image is one of several from the night that haunt Andrew Sutton. For many years, he says, he thought about Tony every single day, blaming himself for not being able to save him. Of the small group at the bows of the Marchioness, Chris Garnham died too, though his girlfriend Gillian survived. In the water, Sutton's instincts took over. "The Bowbelle's side was very close to us and I could hear the thump-thump of the propellers. I knew we had to get away to avoid being sucked into them and I started dragging Helen towards the north shore. Her woollen jacket was dragging her down and I had to fight to pull it off her."

The wreck of the Marchioness was pushed towards the bottom of the river, and survivors described the horror of finding themselves in utter darkness, tumbling against bodies amid churning, coffee-dark water and then, for the lucky ones, a frantic swim up to the surface. Most of those who lived were rescued within minutes by the other pleasure boat, the Hurlingham. However, the Bowbelle carried on, the skipper radioing in the news of the collision, but not even throwing its own life rafts into the water for survivors. For Sutton, the retreating ship was like an image from a film.

"It just kept going. This thing with eyes that had eaten us up and spat us out. It just kept moving away from us, as if it didn't care or even know what it had done. Then there was a soft pfzz-pfzz noise like bottles of lemonade being opened. That was air escaping from the Marchioness. Then I heard other people in the water. Not screaming. It was more like a chirping sound to me, gentle and a long way off."

Unlike most of the other survivors, Andrew and Helen were being pushed rapidly back upstream by the strong tide. Swirling currents and eddies around the bridges kept the people in the water on the other side of the Bowbelle in a fairly localised area. That night the Thames had a three-knot tide, and even the strongest swimmer cannot battle for long against just one knot. Sutton says he saw one other couple being swept upstream towards Westminster, but they were in the middle of the river and were sucked under by the powerful rip currents around one of the bridge stanchions. "I was shouting at them, trying to get them to swim away from the bridge but they didn't hear me. I saw a pair of legs in blue trousers kicking at the surface, and then they were gone."

Fighting to stay afloat and support Helen, Sutton tried to find anything in the darkness he could hang on to. They passed close to other boats and barges moored in the river, but everything he tried to grab was greasy and covered in algae. He closes his eyes tightly and rubs his fingers together, turning down his mouth as if he can still feel the objects in the water. "I remember," he says with a small smile, "thinking as the lights on the South Bank drifted past that I was seeing London from an unusual angle, one that not many people had ever seen. And then being struck by the weirdness of having that thought when I was fighting for my life." After a few minutes he saw rescue boats heading downstream, but they couldn't see him. Like the Bowbelle, he felt they were ignoring him. Once again, the feeling that he wasn't part of the events surrounding the Marchioness was reinforced. "I felt so utterly alone. Why couldn't anyone see us right here in the middle of this city? How could we be so... abandoned?"

Later, near the Oxo Tower, Sutton found something buoyant floating in the river. It was orange and there were ropes or netting attached to it, something he could force Helen's arms under to keep her afloat. She was by now unconscious. After almost half an hour in the water he, too, was exhausted, although he had been able to place her in a proper life-saving hold with one arm around her body.

"I think I had given up then," he says, softly. "I felt like it was time. Time to go to sleep and let go. I couldn't be bothered fighting any more. My head was under water and I had a feeling that everything was fine. But then I saw Helen's legs above me as if she was going up into the sky. That's when I passed out."

Helen was being dragged aboard a police launch. Moments later, Sutton woke up when a policeman stepped on his hand as he lay unconscious in the bottom of the boat. The two policemen on board were resuscitating Helen. As the pain in his hand brought him round, Sutton heard one of the policemen say: "It looks like that one's still going, too." For Andrew, this was another small piece of rejection. He was "that one" - just something plucked from the river, simply a piece of flotsam. He is sure that he and his girlfriend were the last people picked out of the water alive.

He was taken to St Thomas's Hospital, where he discharged himself after a few hours, sitting alone in a cubicle with a blanket wrapped around him. There were, he says matter-of-factly, other people who needed the doctors more than he did. A nurse arranged a taxi and warned him that there were cameras and reporters at the front door. "I went out another way and arrived home feeling as if I'd had a weird dream. I called a friend who drove over and suggested going down to the river to see if we could see anything or help in any way. When we got there I realised he'd taken me to Battersea Bridge, completely the wrong part of the river, but I didn't really know what was happening. There was nothing to see and we came home. Nothing in my head made sense."

The next few weeks passed into a blur of tears, funerals and, for Sutton, terrifying flashbacks of the Bowbelle that made him doubt his own sanity. In an era before mobile phones, the survivors retreated into their own pockets of grief, gradually making contact with those they already knew before the night of the party. Mixed with the horror of the memories, and grief at the deaths of Chris Garnham and Tony Loh Manyem, was the sense that Sutton was alone. A policeman visited a few days later to take a statement, but it was only months later, when the legal wrangling began over who might be to blame for the disaster, that any form of psychotherapy was offered. For Sutton, the therapy seemed irrelevant, and he says he often fell asleep during his sessions.

The aftermath of the Marchioness collision was protracted and messy. Campaigning groups lobbied for an official inquiry into how an accident like this could happen in the middle of a modern city like London, and among the most vocal were Francesca Dallaglio's mother Eileen, and Margaret Lockwood-Croft, who had lost her son, Shaun. A public inquiry into the accident was refused by the then transport minister Cecil Parkinson, and again in 1993 by his successor, Stephen Norris. A later report by the Marine Accident Investigation Board was criticised by relatives and survivors as a whitewash. Eventually, allegations of manslaughter against the skipper of the Bowbelle and the ship's owners were brought, but dismissed for lack of evidence in 1992.

Criticisms of the coroner's office, the crews of both the Marchioness and the Bowbelle, the Port of London Authority and even the lack of preparedness of the Thames River Police were swamped in a complex legal row that continued until a coroner's inquest finally ruled in 1995 that the victims had been "unlawfully killed". Not until February 2000 was a formal investigation launched by John Prescott, then secretary of state for transport. Lord Justice Clarke then reported that a formal investigation should have been held earlier.

Andrew Sutton followed all of these events from the sidelines. He could not escape the abiding feeling that in some way he didn't deserve to be part of the "official" Marchioness tragedy. He continued to deal with the flashbacks and the nightmares, the chronic insecurity and depression on his own. For several months, if not years, he says, he would find excuses not to leave home, certainly not to travel abroad. When he did eventually venture abroad he made meticulous plans, checking and double-checking all of the arrangements to make sure "nothing unexpected could happen". And slowly he rebuilt his career.

The flashbacks never went away completely. They were always the same: the image of the massive black bows of the dredger coming out of the night. They were so vivid that he would have to crane his neck to try to see around the image that dominated his vision.

Sutton says that what he needed most was someone to tell him that his reactions to the tragedy were normal. Although he regained control of his life, he avoided thinking about the damage he had suffered by throwing himself manically into work. "I never stopped, and when the internet arrived it was wonderful, I could work wherever and whenever."

In 2000, by now married to someone unconnected with the disaster, Sutton was introduced to Mary Kayitesi Blewitt, founder of Rwandan Survivor's Fund (survivors-fund.org.uk). She had learned that Sutton had an interest in an eco-tourism project in Tanzania, not too far from Rwanda. As he was travelling to the region regularly, would he be interested in helping to videotape testimony statements from genocide survivors? Without knowing much about Rwanda, he agreed. "Then, I only thought of the practical ways I could help," Sutton tells me. "I didn't imagine that going to Rwanda had any relevance to me and the Marchioness."

But in Rwanda, which he has now visited a dozen times, and confronted with people who had been treated unimaginably cruelly by other human beings, Sutton found that he forged a connection. "I recognise the way the genocide survivors speak about what happened. In Rwanda I am often recording statements in a language I don't understand; but the facial tics, the way people pause, and the way you can see they've gone back to a moment that is truly awful, those are things I understand."

It's a difficult question to ask, but I wonder if there is a scale of horror which makes losing a couple of friends less bad than seeing your children tortured and then slaughtered. Sutton doesn't ridicule the idea. "I think," he says, "that there probably is a scale of horror. But everyone deals with loss and death differently. What I found is that surviving the river made me empathise with people not just because they had lost loved ones, but because I knew what was happening inside their heads afterwards. That's what I wish I'd had - someone to say that they understood, and that I wasn't crazy because I felt anxious, depressed or scared all of the time."

In Southwark Cathedral there is now a memorial to the 51 people who died on the Marchioness. Sutton says it is not his memorial. He isn't religious, describing himself as "mildly Buddhist, if anything". However, he has recently found something approaching his own memorial. It is the Bowbelle itself, the boat he always called "the monster". The dredger was sold to a Portuguese company shortly after the accident, and renamed the Bom Rei in 1990. In May 1996, she broke apart and sank in just over 30m of water off Madeira. Last month, Sutton visited the wreck. He filmed the event, and I went to his office recently to see the pictures.

On screen, he and two diving partners swim down a tethered line towards a murky silhouette covered in sand. In the green Atlantic light they arrive and slowly swim forward. Sutton reaches the pointed bows and, for a moment, hangs at the lip staring downwards at the sea floor. He flips forward and descends the last few metres to sit on the sand, finally facing the deadly V-shape that has haunted his dreams.

"It's certainly the first time I've cried under water," he says without embarrassment, as we look at the footage. In part, having avoided the press for so long, he chose to talk to me about his story because we share a love of diving. He felt that I might be able to understand why going under water to see the ship might close the circle on the events of that night on the Thames 20 years before. As we watch the video of the dive together, I ask him if he can describe what it was like to see the dredger under water.

"I was expecting it to be difficult. I warned my dive buddies to watch me carefully in case I swam off and did something stupid. But gradually, as I swam around the wreck, I realised that the Bowbelle was just a boat. Just a bloody boat. But I still had to go up to the bows and see those 'eyes'. And touch it. I definitely felt something, a physical discharge of..." He pauses. I think he is about to cry, but he just speaks even more quietly and stares at the video images on the screen in front of us for a moment. "Energy, anger... I don't know. In a way I actually felt sorry for the boat because its own life has ended. Now it's just a wreck, but it's becoming a reef, being colonised by algae and providing a home for fish and crabs and snails. It's actually a source of life. God, that makes me sound like a dreadful old hippy, doesn't it?"

Sutton turns blokey, trying to laugh off the idea that the boat still affects him. But I press him on how he really felt diving on to the wreck, because I know how being under water often intensifies emotions. Sutton takes a deep breath: "I have never gone under water without thinking of Tony, and Chris, and how they died. That's what made me cry when I touched the boat. For them, for me, for all of us. For what it did to us, our bodies, our lives, our everything."

Will he want to return to the wreck? "No," he says. "I've done it. In my head I've replaced the picture of the monster with these new images. I know where it is; it's just another shipwreck."

Are the events of 20 August 1989 finally behind him? "Yes," he says firmly. "When I contribute to something like the Rwandan project, I'm hopefully giving something back. That's a small piece of good that came out of that tragedy. But, I'll never lose the memories of the people who were involved. I still talk to Chris and Tony in my head, the same way I speak to my mother and others who are gone. Those memories can be stirred by a smell, a colour or something someone says. But even if I am near the Thames and I start thinking about the accident, it's not with a sense of foreboding any more. Just simple, ordinary sadness."