You don’t have to put your face under to enjoy the benefits of a cold-water dip. But if you do want to, it’s a good idea to do it gradually.

“We want to get our breathing under control first,” Paralympic swimmer and triathlete David Hill explains, as he guides a group of us through an early-morning swim session at an icy south-London lido. “You don’t want to get caught by cold water shock.”

So, after a few warm-up exercises, we wade in and dip up and down to our shoulders for a few minutes, letting our limbs and torsos take the first hit, before braving a dunk to the neck. Hill gives us a mantra – “I am strong” – which I repeat out loud to ward off thoughts of climbing out and legging it towards a hot shower. The water is around 6°C and – even in a wetsuit – bites sharp at my skin, making me breathe in rapidly, and momentarily forget how to breathe out.

This involuntary gasping is a common feature of cold water shock – a key reason why safety is crucial when it comes to any cold water exposure or swimming. You can’t really stop your body from reacting to the cold, but you can train yourself to respond with a sense of control. It’s a “bit of a mind game”, as Hill puts it, and after steadying my breath, I plunge my face under and feel my skull burn. Pleasure or pain, I’m not really sure, but it’s certainly zapped me to life. The pandemic cabin-fever funk still lingering in my bones evaporates, as all my cells zing and prickle.

Cold-water swimming has boomed in popularity over recent years, with people from all walks of life enticed by the promise of an exhilarating – and sometimes life-changing – mood boost. Studies have found it really can alter brain chemistry – and that feelgood effect can happen quickly, with some saying even 30-90 seconds can be enough to clock some benefits.

Triathlon gave Paralympian David Hill, who was born with no left forearm, his first proper taste of cold water. He’d already been swimming competitively for years, becoming the youngest member of the GB squad at the 2004 Athens Paralympics aged 15, before making the switch and becoming a top 10 finisher in the Rio 2016 paratriathlon. Hill retired from competitive sport in 2016 and now keeps busy as a coach, mentor and LGBT ambassador, visiting schools with his motto, ‘there is no such word as can’t’.

Today, he’s leading Zone3’s ‘Urban Winter Swim Experience’, in partnership with LGBTQ+ aquatics charity Out to Swim and Wiggle. I’ve been kitted out with a Zone3 Women’s Aspect Thermal Wetsuit, designed to enable greater lateral leg movement, so you can swim breaststroke or front crawl. I’m certainly grateful for the extra layer, and although no wetsuit completely guards against the cold, they do give you a bit of a buoyancy boost to (hopefully) enable you to stay in the water longer. After some laps and a relay race, we take the wetsuits off to end the session with a ‘skins’ dip (the lingo for going in with just a normal cossie on), then warm up in a Changing Robe with some hot coffee and croissants.

For many cold-water fans, the post-dip snacks and drinks are a big part of the experience. “Sometimes, we go down to the beach and it might be too wavy or there might be a pollution warning, but we’ve all got a hot flask of tea, some cake, so we take the opportunity to sit on the beach for a couple of hours and catch up, which is what it’s all about really,” says Hill, who swims with a group of friends every weekend in Devon, where he lives. “It’s 10 o’clock every Sunday morning and that’s my one non-negotiable. It’s really nice that I’ve got that to look forward to during the week.”

When he first got into triathlon, Hill thought he’d “have the swimming part sorted – but it was a very different sport in the end”. Since retiring, he says he “kept the open and cold water swimming going, as opposed to the pool swimming”, because it’s not just about fitness any more. “At the moment, I’m in the water for about five to 10 minutes with the temperature, so it’s a very limited fitness opportunity,” he says, “so that’s where I really appreciate that it’s more about the community.”

The psychological benefits are a big part of it too, and like many chilly swim fans, Hill has found the effects last long after he’s dried off. He uses those aforementioned mantras to harness “focus and intention” and create a “transfer zone” for being present in his body. “That’s where cold water swimming is quite meditative. When you get into the water, that’s all you’re thinking about. You’re not thinking, ‘Oh, what am I going to have for dinner’, or ‘Oh, I didn’t reply to that email’, you are focusing on your body and your breath, and that’s what I love about it.

“That is the art of meditation, isn’t it? Just being really present in that moment, in your body, how you’re responding and feeling. Cold water really enables that shift and focus to happen, just in a really exaggerated way.”

Hill says resisting the urge to compare ourselves, ties in with the safety aspect of cold-water swimming, as going beyond our personal experience level too quickly could be extremely dangerous. It’s a philosophy he tries to live by in general. “I think it’s wonderful if we can just be proud of our achievements in everything we do. Just be really proud, without having to compare it to other people,” he says.

“There’s a bit of me within sports that’s always tried to do that, otherwise you’ll never be pleased with your own achievements. In a swimming race, for example, if that’s your personal best time and that was your target, and it’s won you a bronze medal, then that’s your equivalent of gold. You can’t be down on that.

“I think that’s really key. Set your own personal goals, and then really celebrate when you achieve them.”

Find Zone3 kit at zone3.com and wiggle.co.uk