Columnist Matthew Parris has been called "ignorant" and his actions "dangerous" after swimming across London's River Thames. But up and down the country, people are dipping a toe in the trend for outdoor swimming, writes Tom de Castella.
It's a sultry July afternoon and about 30 bathers are floating, chatting and breast-stroking their way through the cool, green water. Oak trees, willows and birch overhang the pond, moorhens pick their way across the shallows and a flock of Canada geese swoops down overhead.
If it wasn't for the brightly coloured bikinis and vigilant lifeguards one might almost mistake it for a pagan immersion ceremony. And yet this is Hampstead Heath, only four miles from central London.
The idyllic scene reflects the growing popularity of "wild swimming".
Isabel Robinson, a 20 year-old student has cycled all the way from south London to get here. "It's fun, like being on the beach but in a city - a real novelty. The water's quite cold but on a day like today it's refreshing."
James Townsend, a 32-year-old office worker hurriedly pulling on his trunks, says there's no comparison with a swimming pool.
"I don't like chlorinated water or lanes. I come from the middle of nowhere in mid-Wales and to me this feels like the countryside."
After a hard day working as a windfarm developer, this is the moment he looks forward to.
"As soon as you dive in and feel the water go down the back of your neck, you relax. It's fantastic."
Wild swimming is the name the publishing industry invented in 2008 to kickstart a new literary phenomenon. Two books with almost identical names - Wild Swimming by Daniel Start, Wild Swim by Kate Rew - were published, detailing hundreds of waterholes across Britain.
The term's meaning is not always clear. For some it's simply swimming in rivers and lakes, others include beaches, while some add in lidos - large outdoor pools.
You might throw London's River Thames into the mix, after Times columnist Matthew Parris indulged in a spot of wild swimming on this famous waterway. The former MP wrote about his troubled swim across the Thames - he got swept three-quarters of a mile upstream - in Saturday's edition of the Times. The Port of London Authority fiercely condemned Mr Parris' actions, claiming he could have caused a boat to swerve, risking collision with "another vessel or bridge or pier".
But most wild swimming is done in less high profile places.
People have always swum outside but as more and more indoor pools were constructed in the post-war period, it became a rather eccentric habit. But in the last few years a dramatic resurgence has taken place, Ms Rew argues. Membership of the Outdoor Swimming Society, founded by Ms Rew, has doubled every year since its formation in 2006 and mentions pop up in the strangest places - this year's International Conference on the modernist writer Virginia Woolf, even included a paper on wild swimming.
"I think we've reached a tipping point where outdoor swimming is becoming normal again," Ms Rew says. "There's always been a hardcore of people who never stopped swimming outdoors. But until now they were an ageing group."
In an echo of how surfers behave, a new generation is using social networking sites to plan swims, she argues.
"My enlightenment moment came a few years ago when I planned to go up to the Lakes to swim. Prior to that it had been an opportunistic thing. It's a bit like with surfers - you don't visit somewhere to go to a restaurant or cultural place, you're going there to swim."
So what is it about wild swimming that inspires people to keep diving in?
"There's very little joy to be had in a public swimming pool," she says. "But there's a slightly inexplicable magic to swimming outdoors. As soon as you jump in it feels like the best thing ever - it's the change in physical state. You're floating and get a completely different perspective of the landscape and wildlife. You notice tree roots, swim up to water lilies and see dragonflies mating."
Indeed, in a survey of OSS members, 83% said they swam outdoors because it made them happier and less stressed.
Sandra Moran is an accountant by day but after work swims with friends in Lincolnshire's River Witham.
Frog's eye view
"I used to swim in the North Sea but now river swimming is my favourite. The water's so soft. It's the frog's eye view that [the late writer and environmentalist] Roger Deakin wrote about - you can hear the midges, see the pollen landing on the water and feel the current pushing gently against you."
The rise of river swimming is due in large part to environmental improvements in the last couple of decades, she says.
"When I was young there was crop spraying from light aircraft and the rivers were yellow and thick with chemicals. The farming's less intensive now, there' much more wildlife. Where we swim it's really clear and more than 20 feet deep."
Harry Eyres who swims at Hampstead ponds and writes the Slow Lane column for the Financial Times, agrees that the late Roger Deakin was a crucial pioneer. For his 1999 book Waterlog, Deakin travelled around Britain, swimming his way through streams, lakes, lidos and coves, turning each dip into a meditation on nature and modern man.
"Waterlog was really seminal," Eyres says. "There's always been oddbods enjoying outdoor swimming. But no-one had turned it into a philosophy before. And Roger was a very eloquent guy, quite dashing with an outlaw spirit."
And while most wild swimming doesn't trouble the authorities, that outlaw spirit - some would call it recklessness - is apparent in some outdoor swimmers' psyche.
The legality of swimming in inland water is "complex and unclear", says Nathan Willmott, a partner with law firm Berwin Leighton Paisner, who advises the Outdoor Swimming Society.
There are no set rules for different bodies of water such as rivers, reservoirs or ponds, and each section of water must be weighed up against a number of criteria. According to ancient case law, one can only swim in "navigable waterways", a test that relates to the passage of boats rather than swimmers.
"There isn't really a book to look in to see if a particular section is navigable, whereas with walking you can look on an Ordnance Survey map for rights of way," Mr Willmott says.
Then there may be local bylaws forbidding swimming, which will cover particular stretches of water, although tracking down any applicable ones is a difficult process in itself, he says. There is also the question of land ownership, which may prevent swimmers from getting to the water.
Last month, British Waterways, which manages Britain's historic canals and rivers, advised against taking a dip in its waters, suggesting swimmers "choose somewhere suitable, such as a swimming pool or lido" instead.
As for Matthew Parris's Thames dip, Mr Willmott says a solicitor would currently be hard pressed to say whether such a swim was legal or not. But London's chief harbour master, David Snelson, calls it "ignorant and selfish".
"Basically, swimming in the Thames is equivalent to taking a walk down the M25."