Come in, the water's fine – feel like a hero as the mercury nears zero

You can't beat the thrill of outdoor swimming in deepest winter. No, really...

Parliament Hill Lido’s December Dip (Chris Seymour)

It is traditionally a time for patterned jumpers, roasting chestnuts around the hearth, and hibernating from the bitter cold. Yet across the country, thousands of people are preparing to celebrate Christmas, Boxing Day and the New Year with a freezing cold dip. From Bude to Brighton, Mappleton in Derby to Mundesley in Norfolk, not even blizzards, hail or ice will deter them.

Some will take cold water baths or showers to help their bodies acclimatise. Others go into denial until they find themselves at the water's edge, scared to take the plunge but too proud to back out. Outdoor – or wild – swimming has attracted a huge following over the past decade. Membership of the Outdoor Swimming Society has doubled yearly since 2006, and stands at 15,000. More than 70 mass open-water swims are held annually, with events such as the Great North Swim attracting some 20,000 swimmers. "From the minute you get into water below five degrees you are effectively dying, it's dragging the heat out of your body at an alarming rate," Kate Rew, author of Wild Swim, says. So why do people do it? I joined 400 swimmers at Parliament Hill Lido's December Dip to find out.

Accompanied by a brass band belting out Christmas tunes and swimmers in merman costumes, tinselled swimsuits and 1930s bathing gear, I stripped down to my bikini and prepared to enter the steely London waters and endure two widths. Encouraged by my friends Sarah and Chris, who didn't seem to be backing out of this ludicrous plan, I teetered at the water's edge.

Cold-water swimming has a reputation for attracting hardy octogenarians who think central heating and other creature comforts are for weaklings only. But these folk are now outnumbered by a younger crowd. Many of the Outdoor Swimming Society's members are aged between 25 and 54. Women make up 55 per cent. And many who try it get hooked. Kate, who founded the society, said: "Olympic distance events like our OSS Dart10k sell out almost as fast as Glastonbury."

Chris Smith, 47, from Cheshire, learnt to swim after a spinal injury and has swum in the Norwegian fjords. "Some cold-water swimming can be uncomfortable; your fingers swell up, you lose feeling in your face, but there's a point where you get over it," he said. "Then you start enjoying it. Part of the pleasure is the challenge. The sense of accomplishment is tremendous – like running a marathon. There's an inner glow that makes putting up with the cold worthwhile."

As I entered the water I felt my lungs constrict, and a shooting pain hit the back of my head. I knew I had to get to the other side of the pool quickly. I thought my breaststroke would soon speed up, but as my hands and feet turned numb I found just propelling myself forwards was a victory in itself.

Kate said winter-swimming diehards claim it boosts immunity and reduces cellulite. "But for us occasional plungers, I think the real draw is the fact that it makes you feel so alive," she said. "[You feel] the most immense endorphin rush when you get out. The world – and your life – suddenly loses all its mediocrity and becomes this bright and amazing place, with you loving everyone in it." Members of the Outdoor Swimming Society agree: 74 per cent say it's a tonic for stress, anxiety or depression, while 55 per cent of them say it makes them happier.

Kate is right about the endorphin rush: after completing my two widths and defrosting, I felt high all day. I can see why people get hooked: I'll be returning next year.

For a list of festive swims, see: www.outdoorswimmingsociety.com