Different strokes: discover the joys of outdoor swimming

"You're going in there? You must be mad," says a man with a metal detector and a rusty trowel. He's not the only person to spot me squeezing my limbs into an ill-fitting wetsuit on the grassy riverbank. Couples walking dogs stare, while families on narrowboats crane their necks. "Bit cold, isn't it?" offers a man in a woolly jumper.

Most of us probably associate open-water swimming with goose-pimpled men in their fifties who break the ice on the Serpentine or gamely wade into the Channel on Christmas Day. But a growing breed of swimmer is breaking that stereotype, swapping the chlorine-infused floating plasters of their local baths for the soul-soothing high that comes with swimming among weeping willows and water lilies.

I've come to a stretch of the Thames in Oxfordshire to find out why. My guide is Kate Rew, founder of the Outdoor Swimming Society. "You get used to that kind of response," she says of the man with the metal detector. "I think a third of people don't understand the joys of outdoor swimming." A regular visitor to my local lido, I'm no stranger to the masochistic joy that comes with lowering oneself into freezing water, but while the glassy flow of the Thames looks oddly enticing, I'm beginning to wonder if this was such a good idea.

Passing fishermen and pristine fields of wheat, we finally reach Day's Lock, where the river widens and elegant canal barges queue for a lift upstream. Under Little Wittenham Bridge, the site of the annual Pooh Sticks Championships, we don our goggles and bright red swimming caps ("So the boats can see us," says Kate). A small crowd gathers.

"Three-two-one... Jump!" Perhaps it's the adrenalin (or my wetsuit) but, bracing myself for an icy jolt as we plunge 10 feet into the Thames, I'm surprised to find the 16C (61F) waters more refreshing than freezing. Soon acclimatised, we lower our goggles and begin the two-kilometre (1.2 mile) swim back to our bags, hidden behind a bush. Straight away it's clear that outdoor swimming is to an indoor pool what fell running is to a treadmill. The first thing I notice is the softness of the water. Gustave Flaubert described swimming in natural water as being like "a thousand liquid nipples travelling over the body", and for the poet Paul Valéry, it was akin to "fornication avec l'onde" (making love with the waves). For me and Kate, it's just "really nice" and much more pleasant than the hard, chlorinated water of the municipal pool.

And then there's the wonderful sensation of being carried along, like cycling with a strong tailwind, in this case by the gentle flow of the river. Slicing through the water with a steady front crawl feels almost effortless and we make quick progress. Visibility is low. I can see the bubbles tracing from the tips of my thumbs as my hands stretch out in front of me, but not much else. But with no blue lines along the bottom to distract me, or pool ends to break my rhythm, the poor vision only adds to the sense of hypnotic concentration and I find myself closing my eyes.

After a few hundred metres we pull up for a couple of minutes and begin a gentle breaststroke. The sound of bubbles furiously rising past my ears as I breathe out between strokes gives way, not to the lifeguard's whistle, but to wind in the trees, baa-ing sheep and twittering birds. "You'll find it so difficult to go back to an indoor pool after this," warns Rew.

Rew, 37, first jumped into open water growing up on her father's Devon farm. Years of thrashing up and down indoor pools followed and it was only during a swimming holiday in the Greek Islands that Rew rediscovered the joy of open water. She's now hooked and swims this stretch of the Thames most weekends.

"My mother thinks I'm crazy," she says, "but I knew there had to be plenty more people like me out there." A year ago, Rew, a London-based writer, set up the Outdoor Swimming Society. In 12 months she has amassed 1,000 members and has high-profile sponsorship from a US swimwear manufacturer.

Rew says one reason swimmers are swapping pools for rivers and lakes is better water quality. "Rivers are a lot cleaner now than when I was a kid," she says. Environment Agency reports say that a decline in heavy industry and a billion pound a year investment in improving water quality, our rivers and lakes are the cleanest they've been for 200 years.

But, ducking my head below the surface again I can't even see my feet; the water here doesn't appear at all clean. "We've been conditioned to think chlorinated water is really good," explains Rew. "But water's supposed to be green and have mud at the bottom. Rivers have been cleaning themselves for millions of years." On the Environment Agency's A to E scale of cleanliness, this stretch of the Thames gets a B (" You're only going to get an A on Dartmoor at a spring," says Rew), and more than 90 per cent of UK rivers meet Government targets.

Reassured, I readjust my goggles and set off again. Every so often we encounter boats and I see yet more people looking at us as if they've spotted some species of river dolphin. We soon complete the final stretch. Dragging myself out of the water on to the grassy bank, I feel more invigorated than after any pool swim.

Rew tells me that many people are scared off by local authorities and landowners terrified of being sued. "In the past, anyone who owned a piece of water felt they could be prosecuted if something happened, so they started putting up 'danger' signs," she says. "But often these places are perfectly safe to swim in."

Rew's mission to link up the country's growing band of outdoor swimmers hots up later this year with Wild Swim, a guidebook that will reveal some of the best places to take the plunge around Britain. The society also organises charity swims for Cancer Research. On 1 September Rew will host Breastrokes, where more than 300 swimmers will tackle Windermere in the Lake District.

"Ultimately, I want the Outdoor Swimming Society to be like the Ramblers' Association for swimmers, with clubs up and down the country," says Rew.

For further information or to sign up for Breaststrokes, visit www.outdoorswimmingsociety.com

Dive in: the 10 best outdoor swims

The Thames, Oxfordshire

Before it reaches London, the Thames is perfect for a dip. Get in under Little Wittenham Bridge and meander through the countryside.

The Serpentine, London

For Londoners who want to swim in the wild . Only views of the London Eye will remind you where you are.

Dancing Ledge, Dorset

At the lowest of the ledges that form the rock amphitheatre at this disused quarry lies a crystal-clear bathing pool, cut out years ago for the pupils of a nearby prep school.

Blue Lagoon, Pembrokeshire

The crystal-clear waters of this flooded slate quarry appear a deep blue against the slate bedrock, so it’s popular with scuba divers, too.

Windermere, Cumbria

Britain’s largest natural lake is ideal for swimming since the introduction of a 10mph powerboat speed limit.

Claverton weir, Bath

Squint your eyes and the winding weir could be a waterfall. The gentle flow of the Avon makes it ideal for families.

Cuckmere Haven, East Sussex

Follow the meandering river Cuckmere to the shingle beach, where competent swimmers can head out and marvel at the chalk cliffs.

Carradale Beach, Kintyre

A huge expanse of pristine white sand gives way gradually to the refreshing Kilbrannan Sound, with stunning views of the Isle of Arran.

Sharrah Pool, Dartmoor

After a 40-minute walk through ancient forest to the river Dart, dip under the waterfall and swim down between giant granite boulders.

Helford River, Cornwall

With its mix of warm and cold currents, this brackish stretch of river is a peculiar pleasure.

Chosen by Kate Rew

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