There is no barometer of fitness trends quite like the school gate. It offers a vantage point from which to determine whether the sleekest mummies are heading to hot yoga or Pilates, for a run or a spinning class. It’s the uniform that’s the giveaway — head-to-toe Sweaty Betty, the Lululemon capsule running kit, the Hoka One One shoes or Nike Flyknits, each statements of particular athletic intent. And the latest accessory to be seen sporting? Wet hair.

Damp locks, once just a sign that you were running late for the school run, are now a symbol of athletic self-discipline. They are being flaunted by the growing army of participants in what is likely this summer’s most fashionable workout: an early-morning swim in the nearest lake or lido, river or stream.

Swimming has long been the country’s most popular participation sport, with more than 2½ million adults taking a weekly dip. However, whereas the latest statistics from Sport England suggest that our love of swimming pools is in decline — 729,000 of us have stopped visiting them in the past decade — it seems the popularity of the sport is being buoyed by a jogging-style boom for swimming in the great outdoors.

The hugely popular weekly park run, for instance, has inspired an aquatic equivalent that is now being rolled out across Britain. The National Open Water Coaching Association’s free 400m Nowca Swim events are open to all levels and abilities. Pay a one-off £10 fee for a safety wristband that holds personal details and medical history as well as detecting your location in the water, and you can turn up at any one of the events nationwide. There are 30, and each week the number of venues is being added to.

“We want everybody to be able to swim in a safe, controlled environment and 400m is the ideal distance for newcomers,” says Nowca’s director of coaching, Richard Kiddle. “After each swim, every participant is emailed with their time and the water temperature.”

Longer mass-participation open water swimming events have become the new marathons. More than 200 are held around the country with the largest, the Great Swim series, expected to attract 25,000 participants at five events this year. In its flagship event, the Great North Swim, 10,000 rubber-hatted entrants will take to Lake Windermere on June 10-12; the newly added 10km marathon swim sold out within days of it being advertised. “We weren’t sure if it would take off,” says Alex Jackson, the director of the Great Swim series, “but over 600 people signed up straight away for the longer, more challenging distance”.

In September, the organisers of the London Marathon launch their own version in the form of Swim Serpentine, a two-day open water swimming festival in Hyde Park that incorporates the British Open Water Swimming Championships. Within weeks of opening the entry ballot, more than 4,000 people had signed up for the mass-participation one-mile swim, an overwhelming response that led to an extension of the closing date to allow another 1,500 swimmers to enter.

I love being out there at 6am with the wildlife and the ducks

Perhaps inspired by the Sport Relief exploits of David Walliams, who swam the English Channel and the 140-mile length of the River Thames, and Davina McCall’s heroic swim in Lake Windermere, the middle-aged seem most up for it. Almost half of Great Swim’s participants are in their thirties or older and a fifth are over 45, with the number of women swimmers just outnumbering men.

What makes it so appealing is that the challenges transcend the mere endurance and body-battering you encounter in marathons and cycling events. There’s the murky water, for starters; the cold, the ducks and weeds . . . the unknown.

Kate Rew is founder of the Outdoor Swimming Society (OSS), an organisation that has had its membership grow from 300 when it launched in 2006 to 23,000 this year. “A lot of people fear the cold more than anything,” Rew says, “but actually the human body acclimatises to cold water very well. And lakes, rivers and the sea are rarely as dirty, dangerous or unpleasant as you imagine. Quite the opposite.”

Jack Burnell, who will represent Team GB in the 10km open water swimming at the Rio Olympics this summer, believes it is the mental barriers that set open swimming apart, whatever your level. “Part of the attraction is that it is so hard,” he says. “People who want to push themselves outside their comfort zones are drawn to that and want to overcome their fears of what’s in the water and whether they might be safe.”

There are risks, of course, the cold being one of them. In the UK, water rarely warms to more than 20C and you will need to acclimatise by limiting your time in the water early on, gradually increasing the duration as your body adapts. “The only way to get used to it is to swim more often outdoors,” says Burnell. “Studies on elite swimmers at the University of Portsmouth have shown that our bodies really do retain cold acclimatisation for much longer than we do the ability to tolerate heat.”

The worst thing you can do is to dive into open water having never attempted it before, particularly at this time of year. A few years ago, scientists from Portsmouth and King’s College London reported in the Journal of Physiology how plunging into cold water during hot weather can cause heart attacks even in young, fit and healthy individuals.

Rapid submersion in cold water, combined with holding the breath, automatically activates two responses in the body — the cold shock response that speeds up the heart rate and causes hyperventilation and the diving response that does the opposite to conserve oxygen — that can trigger abnormal heart rhythms.

“As air temperature rises dramatically, people start to go into water that remains dangerously cold,” says Professor Mike Tipton, who runs the extreme environments laboratory at Portsmouth’s department of sport and exercise science. “The body’s responses to immersion in cold water are profound, uncontrollable and can result in drowning and heart problems within seconds. Those wanting to enter the water should do so in a slow and controlled fashion to minimise these hazardous responses.”

Done safely, it’s a far cry from the orderly and contained comfort of a swimming pool. If you can swim, you can swim outdoors, although it takes a period of adjustment whatever your level. “One of the things that first strikes you when you swim outdoors is that there’s no black line to follow, no roof above you, no walls to turn on,” says Burnell. “Through necessity you learn to swim with your head lifted out of the water just so that you can see where you are going and don’t end up swimming in circles. It’s a different experience altogether.”

And it’s far from boring. In a survey of OSS members, 83 per cent said that swimming outside made them happier and less stressed. Sarah Parfitt, a radio producer from Berkshire and a recent convert to lake-swimming, says she can’t imagine returning to chlorinated waters. “I love being out there at 6am with the wildlife, the ducks and the birds,” she says. “It’s so serene and has a real calming effect before I head back to the madness of the school run and rush hour.”

Venues are inspiringly diverse. Richard Kiddle organises daily swim sessions in the London Docks. It’s the first time swimmers have been permitted to enter the water here regularly in 30 years. “The water is a bit salty, but really clean and pleasant,” he says. “It’s tested every two weeks against EU bathing regulations to ensure its purity. We get anyone from City workers to older people coming along.”

For many, the sense of escapism proves the ultimate draw. Rew says that outdoor swimmers are split between those who want to enter mass events and the traditionalists who are content to wallow in the water at their own speed. “Although it is fantastic that outdoor swimming is enjoying such a rise in popularity, I’m not sure I want to swim in a lake with thousands of people, loudspeakers and a helicopter overhead, as you get at some of these events,” she says. “For me, the beauty of outdoor swimming is the surroundings and either the solitude or the company of friends.”

Non-competitive OSS events are regularly a sell-out. The Dart 10km and 10-mile swims attracted 1,600, many of them first-timers, who made their way from Totnes to Dittisham along the River Dart. “Everyone does it at their own pace,” Rew says. “There is no pressure to finish fast. Just to enjoy the experience.”
Great Swim events (greatrun.org); Outdoor Swimming Society (outdoorswimmingsociety.com); www.swims