IT’S early morning. My friends and I stand on the east bank of Loch Lomond, windmilling our arms, eyeing the water, while a nearby group of fishermen stare at us in mild disbelief. Rob, Kris, Pat and I are in trunks and goggles. The “suits”, Jen and Neil, have zipped on their wetsuits, taking a bit of ritualistic heat from us “skins” – those who shun warmth for the full sensory experience of wild swimming.

Then the hats go on and we stride, with purpose and trepidation, towards the jetty and our jump-off point. The question – why are we doing this – is left unsaid this morning, but as usual, it hangs there.

There’s a bit of banter with the guys whose rods poke out from the pier and whose fishing lines stretch into the water we’re about to plunge into – “You guys are crazy”; “No, you guys are crazy”. There’s been friction in the past – we’re about to scare the fish, after all. But this loch and this jetty are public and – fishing licence and bagsied space or not – we all get to share these waters, even if the best days are when it’s just us and the ducks.

So now I’m standing on the edge of the jetty, waiting for someone else to jump first. Early morning light around us and cold black water beneath, and again that question, that nagging doubt that all of us skin swimmers (barring a few hardened veterans) ask: why am I doing this?

It’s a question more and more people are answering for themselves. Last weekend around 2,000 swimmers took part in the Great Scottish Swim, a mass organised race starting from Balloch on the other side of Loch Lomond. They, like thousands of others across the UK, are making the leap from the pool to Britain’s cold waters, joining official clubs or going out in small groups of friends like ours; seeking to enjoy the experience of swimming in lochs, seas and rivers, as far removed from the indoor pool as the treadmill is from fell-running.

But growing popularity or not, standing fearful on the edge of shock and cold water, there’s a part of me that wonders, every time, if I’m crazy, if my need for wild swimming is a form of mid-life crisis, if I wouldn’t have been better practising something safe like yoga, to ease my injured back.

It was after slipping a disc that I’d first begun visiting the pool, having heard swimming was the best pain-relieving alternative to analgesic drugs. Bored with doing repeated lengths and having friends who were enthusiastic wild swimmers, I kept going in order to get fit enough to join them in more natural surroundings. Since then, I’ve twice crossed from Mull to Iona (both times magnificent) and last year took part in the Cumbrae to Largs charity swim, a real challenge.

The jump is both the easiest and hardest part of every swim. Physically all you have to do is lean out, step forward, let yourself go. Dive if you want to. Mentally, however, you have to hammer down your instincts and block out your fears.

We are brought up to fear open water, and rightly so. It’s dangerous and unfeeling and does not discriminate between young children and old folks. Most people learn to swim in indoor pools where the greatest danger is a verruca; where bored guards whistle at bad behaviour; where the water is warm and clear and reeks of chlorine; where the only living thing is you and other swimmers in a big bath, all nicely safe and sanitised … apart from the pee.

Loch Lomond, of course, is neither safe nor sanitised. It’s wild and beautiful, the hills – Ben Lomond, Conic Hill, the Arrochar Alps on a good day – framing the big sky above. Across the water lies the tree-soaked peak of nearby Inchcailloch, one of the loch’s largest islands. A vast expanse of black water surrounds us. On windy days, when white-topped waves promise a bit of chop, the halyards of moored yachts chime against their masts. You learn to read the wind and waves and choose your route accordingly. It’s best to return with the current.

During these first few moments in the water, time freezes. Your senses are on overload; every nerve screams, like a thousand small electric shocks all over your body. The muddy, green-brown waters appear lighter than when viewed from the pier, but still, you can’t see much.

You kick to the surface, lungs going like the clappers as you gauge the temperature. Sometimes if it’s really cold, there’s panic. The next few minutes are critical, especially in water below 14C. You have to start swimming immediately to keep your core temperature up. Though your body is fully immersed and an icy grip claws your limbs, putting your face in the water is painful. Breathing is difficult; doubly so if you are hyperventilating from cold water shock.

Adrenaline kicks in. My body remembers all those lengths of the pool and my feet start to kick and arms rotate. I breathe, reach out, grab a ball of water, pull it down, push it away. Repeat. The face goes in the water for a few more seconds. Repeat again. The gulping for air, frantic and panicky like a landed fish, slows. Breathing becomes controlled. I repeat again and again and before I know it we’re 100 metres from shore and the strokes, kicks and breathing are all coming together.

Endorphins build, and the question shifts from “why do we do this” to ... why would you not?

Sometimes, we are up early enough to catch the sunrise; the first light breaking through the clouds and kissing the land around us, its warmth noticeable and welcome. We have swum in storms and fog ... though breaking the ice is too great a challenge for me. On calm days, the long branches of oak, birch, alder and hazel trees reach out from the island of Inchcailloch, their long fingers dipping into the water, disrupting their reflection.

This morning, a heron stands watching as we swim by. A family of ducks crosses our paths, their feathers getting caught up our wake. Swans lurk menacingly. Often, though, it’s what lurks beneath the surface that focuses attention, our goggled eyes searching for limbs, bodies, tentacles, monsters or, worst of all, pike or jellyfish, momentary distractions from the pleasure of moving through the water.

The physical and mental health benefits of cold water swimming are well recorded. Regular dips, scientists say, can help reduce blood pressure and cholesterol, boost the immune system, condition the skin, improve libido and fertility. Though they may not be lean or in perfect shape, many people report feel fitter and happier after taking up wild swimming. And we rarely get colds.

Karen Weir, who competes in international sub-6C races, says: “Since taking up open water swimming, I’ve seen an improvement in my health. It may or may not be connected but I very rarely catch a cold now and it’s a good stress-buster. No matter how my day has gone or how bad I feel, once I have been in the water I come out refreshed, happy and planning my next swim.”

Weir, who helps organise the Wild West Swimmers community on Facebook, admits she finds the extreme cold water challenging – she took part in a non-wetsuit race in Estonia at 2C – though for her that’s an incentive rather than a drawback. “It would be fair to say I am slightly obsessed with it,” she adds. “There is no better feeling than to be in a loch early in the morning or during winter when there’s hardly a soul around. There are no lane restrictions, no overcrowding, no chlorine. It’s just perfect.”

According to Pat MacLaren, a fellow swimmer and GP, medical studies are increasingly looking at the physical benefits of exposure to cold water. “Recent findings,” he says, “show additional benefits of swimming may come from the exposure to cold rather than the exercise itself. Repeated cold exposure may condition the body into diverting calories from the production of white fat – generally stored under the skin – into brown fat.”

Generally stored around organs, brown fat produces more heat when burned. This, adds Pat, “has obvious benefits for surviving in cold environments though it does this relatively inefficiently, meaning you use up more fat reserves”.

Pat believes there’s a lot of scope for reaping the benefits of this effect, particularly as the country faces an obesity epidemic. “You may burn more calories doing exercise in a cold environment than in a temperature-neutral environment; the theory being that cold exposure triggers a change in the way the body stores excess calories.”

But he warns: “Before people rush off into a cold loch to shed some weight, it’s worth remembering that cold is dangerous and cold water doubly so. Safe conditioning requires repeated graded exposures. Having said that, much as I hate the 6am starts and icy cold water of April, cold water swimming is a wonderful thing to do, great for your physical and mental health.”

Our safety-conscious society would have us believe otherwise. Municipal swimming pools deeper than five feet are becoming rare, and every year, amid grim news reports of drownings in our seas, rivers or quarries, there are fresh warnings to stay clear of the water.

Yet strong swimmers tend not to drown, and of the 338 drownings in the UK last year, only 36 were swimming-related (worryingly, six of those were in a pool). The greatest number of drownings – 138 – involved people who had fallen in while walking or running along a riverbank or coast.

If cold water swimming became as popular as running or cycling, would drownings decrease? Evidence from Germany, which has a healthier attitude to wild swimming (though, to be fair, warmer waters too), suggests wider participation in the pursuit would have no negative impact on drownings, and may even mean fewer.

There are, of course, real dangers involved in wild swimming, and not just from inattentive motor boats. Hypothermia, when it shifts from mild to moderate, is scary and serious. But, as with climbing, calculated risk is part of the fun; the challenge of being in an alien environment is compelling and thrilling. Yet the risks must be taken seriously. There is no safety net on the loch.

Here in Loch Lomond, we swim close for safety’s sake. Occasionally, fear strikes; the mind can play tricks. Variations in water temperature have you wondering about the onset of hypothermia. Half-submerged branches become the limbs of sea monsters; wet leaves stick to you, and weeds seek to wrap their tendrils around your ankles.

We stop to tread water, each looking round to check the others are fine. Strangers until a couple of years ago, we now rely on each other for our safety. A quick catch-up; a bit of laughter.

Why do we do this, I ask? The answers are quick fire. Because it’s here. Because George (our coach) tells me to. Bad back, hate pools. I just love it. But the clock is ticking, as my body reminds me. These conversations are marked by their brevity. And so the swim resumes.

After an hour or so we return with the current, those same waves we had to slog through now guiding us back to the jetty.

On shore, the fishermen are gone and the car park is deserted. I can feel my core cooling and I need to get warm. There’s no-one to witness our bodies emerging, cold, shaking, yet glowing from the euphoria. We have to move quickly here – even out of the water our core temperature will still be dropping. Numb fingers hinder the removal of swimming caps. Pants can be a problem, socks a nightmare. But amid the shivering jaws and stabbing pains in the feet, we are all grinning, all buzzing, reflecting on today’s swim and already looking ahead to the next one.

Why do we swim? George, our coach, sums it up best: “It makes you feel alive.”

The Cumbrae to Largs charity swim, an annual 2.5km sea swim in the Firth of Clyde to raise money for Gillian’s Saltire Appeal, takes place on Saturday, September 19. Closing date for entries is this Friday