You don't have to be mad to swim here ...

A slice of Britain: Throughout December normally sane people will leave their cosy homes to head for a stretch of water to partake in the growing sport of festive outdoor swimming. They'll freeze, but claim the life-affirming glow is worth it

Bracing doesn't begin to describe it. It's a cold December morning in London, the ground is still wet from the night's downpour and I've just dived into water only two degrees warmer than my fridge.

The sane among you will ask, "why?" It's a good question, and one I had pondered from the warmth of my bed in the early hours before taking the plunge, as the rain lashed against my bedroom window. It is hardly bikini weather. The thing is, some people do this for fun.

The Plum Pudding Plunge is the first organised outdoor dip of the festive season, with about 240 people braving the chilly conditions to swim in Parliament Hill lido. Organised by the Outdoor Swimming Society, the annual London event involves people "with just enough lunacy" swimming two laps (60 metres) of the lido. The odd "frigidly gifted" person manages a bit more. The camaraderie is wonderful.

This weekend and for the rest of the month, up and down the country, thousands of equally hardy souls take to outdoor pools, lakes, rivers and the sea to celebrate Christmas and New Year while the rest of us laze in the warmth, stuffing our faces and nursing hangovers.

"It makes you feel alive," explains fellow dipper Amy Smith, 22, who has travelled down from Swansea, where she is studying to be a midwife, especially to get wet and cold. Fifteen of her university friends were meant to join her, but wimped out one by one. Amy and her family swim in the sea at Sennen Cove, Cornwall, every Christmas. Her cousin, Zoe Birchenough, 36, swims in the Serpentine in London's Hyde Park several times a week. "It's a beautiful place to get hypothermia," Zoe says. "It's spiritually uplifting."

As I arrive at the lido, stripping down to my swimming costume, strangely, doesn't seem such a crazy idea. The sun is shining. A group of people are serenely practising tai chi at the side of the pool. Happy-looking swimmers are effortlessly ploughing up and down. Heck, there are even a few palm tree-like shrubs dotted around.

But then it comes to removing my layers of wool jumpers, coat, hat, scarf, gloves ... and I start to lose feeling in my toes. Suddenly I'm wondering if there is any lard or goose fat doing the rounds. Nope: these guys are hard.

Patricia Baker, 69, of Saffron Walden in Essex, her daughters Melissa, 43, and Susannah, 42, and 14-year-old granddaughter Eloise will be tackling far chillier waters at the world Winter Swimming Championships in Bled, Slovenia, in January. "We are mad," Patricia admits. "It's addictive."

Following motivational talks by Owen Sheers, the poet and outdoor swimming enthusiast, and Ben Fogle, the TV presenter and Antarctic adventurer, we line up and, well, take the plunge.

Fogle, whose wife Marina is due to give birth as we speak, reassures me about the swim: old wives' tales about people having heart attacks after diving into cold water are just that. "It's about controlling your breath," advises Fogle, who wants to do more outdoor swimming and is using the event as "research" for his next adventure with the Olympic rower James Cracknell. "It's probably a bit like childbirth!"

The shock of hitting the water certainly kicks my heart into action. The society's website warns the water will be "so cold it'll drive your testicles into your throat". I don't know about that, but it is freezing. According to regular sea bather Amy, it is "much colder than the sea". The (lack of) temperature takes my breath away and I'm left floundering for a few seconds as my arms and brain fail to link up. It gets easier as I get into something resembling breaststroke on my second lap but I can't wait to get out of the pool and into the specially erected hot tub.

This year's December dip at Parliament Hill attracted three times as many swimmers as last year and, three years after its formation, the Outdoor Swimming Society boasts 7,000 members. And with the actor Robson Green's Wild Swimming Adventure starting on ITV1 this week, plunging into cold water is suddenly cool.

According to those regularly swimming our rivers and shoreline, the growing popularity of triathlons and our increased awareness of the outdoors is helping to attract people to the open water.

"I do go to swim training in indoor pools but it's like running on a running machine," says Michael Worthington, 39, of London, who regularly swims in the Thames. "It just seems a bit pointless. Part of the joy of swimming is where you go and what you get."

Swimming down the Bosphorus allowed him to see Istanbul "from a completely different perspective, from a view most people who live there haven't seen it".

The society's founder, Kate Rew, adds that outdoor swimming makes "you feel so happy afterwards. It's such a life-affirming thing to do. There's also that slightly renegade thing to outdoor swimming. Nobody's really sure they are allowed. There's a general perception that it's difficult and dangerous, but we are trying to change that and it's open to everyone."

Cold water swimming, I'm informed, is also good for your health. As well as giving you a natural high, it boosts your immune system, gets your blood pumping, improves your sex life and burns calories.

As Fogle says, there is something great about taking yourself out of your comfort zone every now and then. Even so, I can't say I'll be rushing back into the water any time soon.