I wouldn’t say it was extremely comforting when, seconds before my first attempt at cold-water swimming at my local pond, as I hovered near the steps, my skin chill against the air, the jovial lifeguard started to tell me about sudden-immersion syndrome. “People can die, because they go into shock,” she explained, eyes focused on something in the distant trees. “But you’ll be fine. Just don’t panic.”

In the summer, I jump into the pond with the enthusiasm of a labrador – ears flying, paws spread skywards. (This was especially the case during last summer’s heatwave, two months of feeling conflicted – the utter joy of that slightly grassy smell of warm forearms, tempered by the fact that the planet is, well, screwed.) Summer swimming is all about the refreshing feel of hair slicked back from the forehead after hours of enduring a sticky, matted mess. Or the tessellating, shimmering patterns of light at the bottom of a hotel pool.

Cold-water swimming is like being plugged into the mains; there is the sensation of a sedentary life spent looking at screens cracking wide open. It’s a body slam-dunking in every department: heart, lungs, brain. The first time, I thought of Jack and Rose in Titanic, as she considers jumping overboard and he tries to talk her out of it: the water will “hit you like a thousand knives stabbing you all over your body”. (Incidentally, has there been a more handsome man than Leonardo DiCaprio in Titanic? This is a rhetorical question.)

Jack isn’t wrong, but knives have their uses. Knives can cut binds and set people free. At times I feel so bad that there is a restless ennui, a nihilistic velocity – every human atom bares its teeth; the shock of the cold water takes the edge off that inner violence. There has even been recent research that cold-water swimming can help with depression and anxiety, by improving the stress response.

I’m no expert swimmer. I wear the same thing in all temperatures – a humble bikini. It’s from Topshop – or somewhere similar that might pay its taxes and isn’t owned by Philip Green. There are stronger swimmers who go more often than I do, kitted out in wetsuits and special waterproof thermal gloves and hats.

But even when it’s deep winter in the city and I look as if I took a wrong turn on the way to the Costa del Sol, I feel a sense of pride when I’ve met the challenge of 5C scrawled on the chalkboard, as satisfying as a maths equation solved. “Reinvigorating” is an overused word in an era of self-help books and journeys to faraway continents to find oneself, but cold-water swimming is truly that. And best of all? It’s free.