Through my aircraft window, the river resembled a snaking green lava flow. A couple of days later I’m swimming in it, a wetsuit protecting me from the 13C chill. The Soca is one of the many pristine rivers in Slovenia, an Arcadian land of lakes, mountains and forests that feels like an unspoilt and less uptight Switzerland.

Our party of seven left the riverbank in two groups. I was in the second and found myself directly behind Martin Strel, probably the most famous Slovenian alive, a paunchy bear of a man in his late 50s who has swum the length of some of the world’s mightiest rivers, and whose feats were the subject of the award-winning 2009 film, Big River Man.

Demonstrating the skills that helped him conquer the Danube, Yangtze, Mississippi and Amazon, Strel leads me through a semi-submerged tree before disappearing off ahead. I’m left alone with my thoughts and the emerald water. My mouth is open, for when a river tastes this cool and fresh it’s hard to resist letting it wash over your taste buds. There’s no discernible current and at one point it’s so shallow I have to stand up, allowing me to see the green, pink and yellow caps scattered ahead and behind.

Apart from Strel, my fellow swimmers are all British – a mother who has whisked her daughter away after A-levels, an ex-policewoman and her partner who works in finance, and another journalist. We are on holiday with Strel Swimming Adventures, a new tour operator launched by Martin and his son Borut.

After a few bends in the river I catch up with the others. Strel is treading water, chatting to some lads in a boat. He is famous for accompanying swimming with Cvicek, a light Slovenian rosé. During his 14-hour Amazon day, he would drink two glasses with lunch, two in the evening. Now the boys beg him to accept a beer and to their delight he takes a few sips, before handing the can back and swimming to our exit point.

A decade ago the idea of going on a swimming holiday would have seemed like a niche within a niche. But today outdoor swimming – in lakes, rivers and the sea – is a fast growing sport, helped by the triathlon craze and a torrent of books and magazine features on “wild swimming”. It is even possible to book island-hopping trips to Greece, Turkey, the Scillies and the Caribbean that rely on swimming instead of yachts or ferries.

Up to now the British firm Swimtrek had a near monopoly on such holidays. Borut used to work as a guide for Swimtrek but decided he was capable of going it alone. So far his start up has only this three-night trip to Slovenia and a week-long tour of the Croatian coast. But there are plans to expand to Austria and eventually the US.

Arriving on the first afternoon was a spectacular experience, our hotel overlooking Slovenia’s largest lake, Bohinj, which we are to swim on the final morning. The group dinner and briefing is polite and a little stilted. But the following morning Lake Bled lifts the spirits. It’s a photographer’s dream – turquoise water, a castle on cliffs above the water, mountains all around, and in the middle an island out of which rises a tall white church tower.

We are given a quick swim test to assess our speeds and help our escorts keep track – there will always be three craft on the water to check we’re safe. The water is warm enough – 18C – for me to do without a wetsuit and we follow the line of the shore as it zigzags around towards Marshal Tito’s old summer villa. Then we cut across to the island for some elevenses and a look at the church.

Borut films us one by one with his underwater camera – he’ll later give us tips on our stroke – and as we’re preparing to leave the island half a dozen boats arrive, disgorging a wedding party from Australia. We stand chatting in our speedos to the immaculately turned out guests, two tribes meeting on neutral territory. The return swim has a dreamlike quality. Strel appears at my side and we swim a metre or so apart almost until the shore. It’s remarkable to think this man took just 66 days to swim the Amazon, a distance of 3,273 miles. Underwater he looks both comically tubby and, like a turtle or a seal, at one with his marine environment.

The following day we’re up early for the drive to Italy, during which Martin explains his reliance during long swims on a silver pendulum that gives him news of positive or negative forcefields. (Later he’ll give me a bizarre list of approved foods and argue that if everyone lived like him, the hospitals of the world would be empty.) We swim half of Lago del Predil, which at an altitude of nearly 1,000m is icy on the feet, surrounded by interlocking spurs and flinty peaks. Afterwards we cross back into Slovenia for gnocchi and salads in the white water rafting centre of Bovec. In the afternoon we hike up to a secluded waterfall before swimming down the River Soca.

The constant activity has helped the group to bond but the itinerary can sometimes feel a bit intense, albeit less so than Swimtrek trips I’ve been on. The last day is the big one – the 4km crossing of Lake Bohinj. Shaped like an elongated panther, the lake is dark green, perfectly still and appears absurdly long to be navigated by front crawl. I start in the middle group and soon move from warm, transparent shallows to cold, deep water. You can’t see much with your head underwater apart from the odd shoal of sun flecked fish. So I pause from time to time to take in the rugged peaks of the Julian Alps, the wooded slopes and lush pastures. It’s easy to be dismissive about “activity holidays”. But when the setting is this idyllic, swimming in the open air is a purifying experience.

Isabel, the school-leaver, catches me up, we exchange pleasantries as if bumping into each other at the bus stop, and then she hurries on. I’m not tired but the cold water is tightening my calf muscles. If this was a movie I’d be focusing on Strel’s mantras about mental strength. Instead I hear his list of approved foods: “Fresh bread crazy for stomach. Lamb, rabbit, goat is good. Beans, onions, garlic, fresh carrot…” The bathos brings a smile to my cold lips. And when I reach Borut’s rowing boat there is a cup of Martin’s recommended honeyed tea. We’ve been in for an hour and 15 minutes and I’m three-quarters of the way home. The hot tea courses through my body and the Kinks’ “Ape Man” pops into my head, driving me to pick up the pace.

There’s drama on the shore when I finally make it in. One of our number is in the shallows, immobilised with serious cramp. It takes blankets, tea and lots of stretching before she can move to the beach. “That was more painful than giving birth,” she says, laughing with relief once the pain has gone. It’s a far cry from that stilted first dinner. Swimming and Slovenia have managed to overcome even congenital English reserve.

This piece appeared in the FT on July 29 2011

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