Cold swimming: Come on in, the water's icy

The snow has packed ice-hard, a freezing wind snatches the breath and the sky is a relentless iron slab. Only a fool would take off all their clothes in these conditions. I am that fool, and I'm not alone – I'm one of many winter swimmers, Tooting Lido in south London is my unheated water of choice, and I'm in "training".

Forget for a moment the hours of sweat and pain that "training" suggests. "Acclimatisation" is a better word here, and "are you mad?" the most common question, because the only way to "train" for the Fourth UK Cold Water Swimming Championships is by getting in cold water as often as you can. For organiser Margy Sullivan this is "a celebration... everyone glad to find like-minded people and share in the joy of the sport". Yes, she said "joy". The first championships, Margy says, brought winter swimmers "out of the closet – it seemed there wasn't a corner of Britain that didn't have a hidden gang of swimmers who surreptitiously dipped in winter". And not just in Britain – there are entrants this year from as far as Kazakhstan, all making what swimmer Lucy Petrie calls "an incredible northern hemisphere community". Around 400 people will emerge to compete in races like the 30m breaststroke, and if that sounds like a breeze, please factor in that the water may, if we're lucky, hover around the 4C mark.

The air temperature is currently -1C, but the water is a comparatively balmy 3C, and I'm getting in it. Standing on a bit of manky towel to avoid contact with the cold floor, I quickly put on a costume, my "cheat's" neoprene boots and gloves (not allowed on race day) and two swim caps, which are better than the obligatory one. Sometimes I wear a woolly hat, too – it's not the look for a first date. The man who ran a marathon in the Arctic wearing just his pants used visioning techniques, picturing a fire burning in his belly. Lewis Pugh, the cold swimmers' god (it's not just me) can raise his core body temperature just by looking at cold water. I try to keep my brain in neutral, because the minute I start to think about facing what someone has called "liquid death", I could easily think myselfout of getting in.

I get into the water with no shenanigans. You can't dither, and neither should you dive, or you could go into cold shock. On race day, I'll have to get in and wait, one shoulder under water. It's a grim prospect, but Margy points out that "there's a lot of peer pressure if anyone hangs about". It takes a few seconds of horrid operatic screeching and aqua-jogging until I can just about bear to lift my feet and push forward. An icy cold washes up my back. My instinct is to gasp and pant, but that can make you feel panicked, so I do controlled long out-breaths, like a whale in labour. I swim a width breast stroke, trying to keep my face up; the water feels somehow both cutting and viscous, it's heavy, I'm slow. I turn, and that 30 metres back has never looked such a long way. Brain freeze is creeping in, a dull hammer at the back of my neck, and my chin is blue-purple and anaesthetised – I may be dribbling, I have no idea. At the steps again, my hands feel too fat and clumsy to grasp the icy stair rail; I have the menacing tingle of sharp pins all over my skin, my teeth are frozen, I'm sunburn-red wherever the cold has touched me, I can't work my fingers to pull my goggles off. Time to get out, time for what Petrie identifies as "the brief moment you feel invincible".

Did I mention I do this by choice? Why? There are as many answers to that as there are cold water swimmers, and there are plenty of those round the country, often only visible when they're taking part in Christmas swims. Anecdotally, winter swimmers claim to have fewer colds, but for fact-seekers, the Outdoor Swimming Society documents some proper health benefits. It improves your immune system and your circulation, as the blood rushes to protect your core, then goes back to your fingers. It kick-starts your body's pain-killing endorphins, and boosts serotonin and dopamine, which, put simply, help make people happy. Yes, I can report, we do seem like a happy bunch, laughing like drains in the sauna afterwards. And it burns calories, so you can eat a lunch entirely disproportionate to the very small amount of exercise you've done. Double flapjacks all round.

Health benefits are a good justification after the fact. But it's a rare person (I'm loathe to use the word "odd", given the context) who thinks, "my circulation is sluggish, I must get into this frozen lake". And there are risks – cold water swimmers have to be alert to the real dangers of hypothermia and shock. What it does do, and no happiness index can measure this, is make you feel alive, and, most crucially, glad to be. Petrie clearly cherishes the "life-enhancing qualities" of cold swimming. "It gives you a good energy," she says, "for the rest of the day. It is to be savoured." Others talk about the "inner zing," the adrenalin rush, and Margy uses a common poolside word – exhilaration.

Taking a dip during the festive season is a tradition for many hardy souls, with group swims on Christmas Day, Boxing Day and New Year's Day now a fixture at many sites around Britain, including Portsmouth, Brighton and Guernsey. The New Year's Day Dip at Peel Beach on the Isle of Man has been running for more than 20 years, with hundreds of people taking the plunge into the Irish Sea every year. Fancy dress is an important element at many events, and nursery rhymes are the inspiration at the New Year's Day dip at Abersoch beach in north Wales.

I hobble-run on frozen feet to the sauna and warm up with people with whom I probably just have this one thing in common. As Petrie says, "the lido is a great leveller, everybody bound by cold water regardless of who you are". And I'm as likely to be sat beside a doctor as a care-worker, an actor as a plumber – though sometimes it feels like everyone is Arts Council-funded. (I say "is", obviously I mean "was".) Conversation isn't restricted to swimming, but invariably revolves around it, though talking about how far you've swum, particularly with the temperature so ridiculously low, is not encouraged because when people feel they "ought" to do more, it can lead to poor decision-making. And if someone's shivering too much or looking for support and encouragement, this is a good place to be.

There is something definitely addictive about all this wellbeing, community and cold water. I'm always thinking about the next time, about how to fit the lido around family festive commitments (other festive cold swims are available). During the championships, Margy tells me, "everyone is urging on everyone else, wishing them to succeed, and revelling together in the achievement". People like me, who used to hate swimming because it's impossible to smoke at the same time, don't often get to revel in physical achievements. That's going to be something to smile about, I think. Though I won't feel it, my face will be frozen.

The 4th UK Cold Water Swimming Championships are at Tooting Lido on 22 January 2011. To enter, visit www.slsc.org.uk. The Outdoor Swimming Society ( www.outdoorswimmingsociety.com )

How to take the plunge

* Eat porridge: it helps to have a slow-burning carb in your belly, so eat a big bowl of warm stodge before you get in. Not just before, obviously, that could get messy.

* Layers: wear lots; ski thermals and cashmere are apparently best, but if you can't do quality, do quantity. Both together is optimum. And wear a hat. Only take it off when you have to.

* Be warm first: build up a sweat with a good brisk walk, cycle, or run beforehand. It makes the initial temperature contrast shocking, but after that, being in the water feels easier – or less hard.

* Be quick: the worst bit for me is taking my warm clothes off on a freezing day. And when you've changed, don't employ delaying tactics ("Ooh, look at the sky"). Just get in the water.

* Don't stay in too long: as if. The body loses heat much more quickly in water. Once in, don't feel tempted to test your limits. It's not the right place. Hypothermia really is dangerous. Keep it short: one width is not shameful. No one is judging.

* Warm up slowly: a warm shower might feel good, but it brings blood to the surface of the skin when it needs to be protecting your core; that can leave you feeling faint and getting colder. Get your kit back on, and drink a lovely hot cup of tea.

* Don't jump: please don't just jump into cold water because you've read the above. You can get into trouble very quickly. Swim with others, preferably somewhere with lifeguards if you're inexperienced, or with other regular cold water swimmers who know what they're doing.