There's nothing like seeing off winter with a cold plunge into the sea - or so says Kate Rew, author of Wild Swimming. Journalist Kate Hodal joins her for a bracing swim in Scotland as they look at the joys - and benefits - of swimming in Britain's lakes, streams and, yes, seas.

It's a bright, sunny March day in Scotland and I'm standing at the edge of the sea, wearing nothing but a bathing suit and swimming cap, clutching an inflatable plastic daffodil.

The water might be sparkling in the sunlight, its gentle waves lapping at the shore, but for the next five minutes of my life, the sea will be my arch-nemesis.

I - along with 49 other crazy people - have come to North Berwick, East Lothian, to commemorate the first day of spring with a bracing ocean swim.

Ranging from the wee small age of five, right up to 60, and all with a mad glint in our eyes, we - students, barristers, filmmakers and grandmothers alike - are taking part in 'The Big Dunk', an event organised by the Nairn's (of oatcake fame) Natural Woman campaign, which is promoting natural ways to vitality such as outdoor swimming.

The water is icy as we rush in. Swimming caps - mandatory to keep the heat of our bodies as intact as possible - bob around as we fling ourselves into the 7C sea, shrieks of pleasure and pain echoing off the water. The braver swimmers break into front crawl or breaststroke; the others - when not gasping for breath - tread water and chat to stay warm.

I grew up swimming outdoors - in the creek that runs past my parents' garden in California and in the Pacific Ocean - and have swum everywhere on my travels, be it in Devonshire streams, the Dead Sea or the Amazon River. But I have never swum anywhere so cold.

My thighs and shoulders start tingling, then burning, as the cold makes my blood dive into the core of my body. Three minutes later, just when I'm not only getting used to it but enjoying it, we get out - the danger of our limbs and muscles weakening in this temperature too great for us to stay in any longer.

We might all be shaking immediately afterwards, but we're smiling too - a common appearance at outdoor swims, says regular swimmer Rachel Smith.

"Wild swimming makes you feel acutely alive," says the family support worker for a cancer foundation, as she wraps up in a towel.

"It allows me to relax from a hard week and dissolve my worries into the water. And the best thing about it is that, unlike other sports, you don't need a natural ability to do it - you just need to enjoy swimming."

Outdoor swimming might be good for the brain, but it also has numerous physical benefits, says Kate Rew, founder of the Outdoor Swimming Society [www.outdoorswimmingsociety.com] and ambassador for the Nairn's Natural Woman campaign.

"In general, swimming gives you a massive boost to your circulation and immune system - but with outdoor swimming, the cold water gives you an endorphin rush and makes you feel high and happy afterwards."

Rew has swum all over Britain, from Scottish lochs to Cornish seas, in temperatures as cold as 0.5C. She grew up swimming in a river on her parents farm in Devon and her book, Wild Swim, charts the many swimmable pools, lakes, creeks and rivers across Britain.

"There's something magical about swimming outdoors and absorbing the world around you," she says, having chosen a stripey bikini for this occasion (while others are in wetsuits).

"It's a much more different experience to swimming in a chlorinated box."

Brits once learned how to swim in Britain's lakes and rivers, and even just a century ago, outdoor swimming clubs and lidos were popular pastimes. But all this changed with the introduction of the municipal swimming pool and resulting health and safety regulations, says Rew.

"When swimming pools became popular in the 1940s, they were considered a luxury, and pools became the 'nice way' to go swimming instead of down in your local river or lake.

"We've lost touch with swimming outdoors ever since. Pollution in the 1950s and 1960s put people off, but the UK's rivers and lakes are clean for swimming again - so why aren't more of us out there?"

Since she founded the Outdoor Swimming Society in 2008, the number of outdoor swimmers has risen exponentially all across the UK, with wild swims organised on and off-season (autumn to late spring) on their own and at events such as the Keswick Mountain Festival or Port Eliot Literary Festival.

But outdoor swimmers still have a long way to go before gaining the ground Rew would like to see.

"The health and safety angle of signs around lakes saying 'Danger: Deep Water, Swimming Prohibited' has made people think that outdoor swimming is illegal. But it isn't," she says.

"Essentially, we want the same freedom as mountaineers or hikers. You wouldn't think about roping off the Lake District and saying, "Danger: Hill", but for some reason people have an extra degree of fear because they're swimming."

Legally, rules regarding outdoor swimming are grey and can change from county to county. In general, you have the right to swim if the water is 'navigable', so if there are boats on it you should be alright. In Scotland, however, there are much wider rights to swim - where a cold dip in the North Sea at New Year's is seen a national pastime.

Wherever you choose to swim, though, you need to keep your wits about you, says Rob Fryer of the River and Lake Swimming Association [www.river-swimming.co.uk].

"Much is made in the media and by so-called water safety experts of how the water 'never warms up in the UK', and how this exposes you to the risk of cold shock, hypothermia, cramp and heart attack.

"No doubt all this is true, but for healthy swimmers, the main risk is from gradual chilling [of the body]. Research confirms that drownings associated with cold water are usually caused by swimming impairment.

"Although at water temperatures of 20C and above, often experienced in open water during the summer, the risk diminishes. Our advice is: Wear a wetsuit, swim parallel to the shore and know your limitations."

Of the 50 swimmers at the Big Dunk, only two of them are men. One is a regular outdoor swimmer (who trains in Loch Lomond), the other my friend James Reid, who's visiting from California and never contemplated the North Sea as a viable swimming hole.

"I loved it!" he tells me later.

"It was weird, different, exciting and uncomfortable all at once. There's something really valuable about testing yourself even just for a minute, feeling your body in a new way and remembering that you're still a human being."

Neither he - nor I - can wait for our next wild swim - and are seeing the wild waters all around us in a whole new light.

Top tips from Kate Rew on outdoor swimming 1. Get inspired: "Research some places to swim outdoors - the OSS is a good place to start."

2. Dress for the occasion: "Short dips in cold water mean you can wear a bikini. For longer swims, you should consider a wetsuit to keep your muscles warm."

2. Exhale before you get in: "It'll allow your breathing to become more regular once you're in."

3. Don't expect to look forward to it: "Even people who swim outdoors regularly doubt they'll enjoy it when they're standing there on the bank, so take a deep breath and plunge in."

4. Focus: "Focus on a tree or a rock to keep you thinking beyond the cold, then take a look around you and enjoy yourself."

5. Dress warmly when you're out: "Put on a woolly hat, gloves, thermals and socks and have a hot drink. If you're properly cold, take a hot bath, not shower, as a hot bath will allow you to absorb the warmth."

6. Do it with people: "Even if the only thing you have in common is outdoor swimming, you'll make friends and get swimming more often."