It took 18 months for the scars on Adam Walker's belly to fade after he was stung off Hawaii by a Portuguese man'o'war.

The open water swimmer has a fund of stories about gruelling experiences on the high seas but Hawaii counts as the most hair-raising. He was crossing the Molokai Channel, between the islands of Oahu and Molokai. He knew it was renowned for its rich wildlife and wore an ankle bracelet that emits a signal to ward off sharks, but because he swam at night in the pitch darkness (a necessity in order to catch the right currents), he did not see the tentacle of a huge jellyfish, which strafed his abdomen.

"Imagine being scalded by a kettle but times 50. It was horrific," he says. "I pulled two tentacles off my stomach. The poison is 75% that of a cobra. Some adults have died of it." Walker was determined to keep swimming, but he lost feeling in his spine and suffered punishing nausea. Then his breathing became laboured. At the same time, he feared being stung again.

These famous jellyfish drift through open water and their 10-metre tentacles collide with and paralyse prey, and though in the event he did not meet his attacker again, he did cut his leg on rocks, causing it to bleed, only to see a tiger shark five metres away. When he finally came out of the water after a 37-mile journey, he was in a bad way and remained so for a week. "I had about seven showers. Sitting in the shower was the only thing that soothed it, because the stinging cells remained activated.

"But it's the ocean; I know the risks. If it were easy, everyone would be doing it. I try to keep a clear mind and say, 'It's one arm in front of the other'."

Walker, 35, from Nottinghamshire, will be appearing next weekend at the Global Open Water Swimming Conference which takes place in Scotland this year for the first time. The two-day event, at Mount Stuart on the Isle of Bute, will feature talks, a 2km open water swim in St Ninian's Bay and an awards lunch.

Scotland has some of the best terrain for the sport in the world. Vigour Events, which organises sessions with coaches and health and safety staff in attendance, expects to clock up around 3200 swims in 2014, in locations such as Bardowie Loch, Loch Lomond and Strathclyde Park.

Walker has swum several times in Scotland, but over the last six years, has focused on achieving another goal: becoming the first Briton to complete the Oceans Seven Challenge, marathon swimming's pre-eminent global challenge. He finally completed the seven crossings on August 6 with a traverse of the North Channel between Northern Ireland and Scotland.

It all began in 2007 when Walker, who had swum competitively as a teenager, watched an in-flight movie called On A Clear Day, a fictional story about a man who swims the English Channel. Walker, then 27, felt inspired.

Back home, after some pool-based training, he ventured into a local boating lake. It was October and the water was nine degrees. "Because I was very naive at this stage, I got into the water and started to hyperventilate. I had on a 2mm-thick vest suit. I put my face in the water, did 1km, then lost the feeling in my feet, arms and hands and started to get paranoid."

After emerging from the lake, he had a warm shower but after 40 minutes his body temperature was 34.5 degrees - anything under 35 degrees is hypothermic.

"It was one of those experiences where I thought, if I'm daft enough to do that, then I'm going to do this properly."

The following year, he started swimming in the sea off Dover. He first crossed the Channel as part of a four-person relay team but in July 2008, swam it solo in 11 hours and 35 minutes, vomiting with seasickness about 20 times during the crossing. "When I arrived, I said to myself, this is it, I don't have to do this again. That lasted about 10 minutes, then I thought, 'What next?'"

"Next" was swimming the Gibraltar Straits. Walker had dolphins flipping out of the water in front of him and could hear pilot whales calling. As a fundraiser for the Whale and Dolphin Conservation Society, this was a particular thrill.

At that point, Walker learned about the Oceans Seven Challenge and, having already done two of the challenges, decided to take it on. The next event was Hawaii and then, three months later, a 21-mile crossing between Los Angeles and Catalina Island. In 2013, he swam Tsugaru, Japan, a 16-mile crossing in very tricky waters where the tides changed every minute. As a swimming challenge, it was the hardest event of all.

Then came an unexpectedly magical experience crossing the Cook Strait in New Zealand, in April this year, when Walker found himself apparently being protected from a shark by a pod of dolphins. He looked down to see the shark in deep water below him, but the dolphins stayed with him for an hour and a half. Even when he swam to the support boat for a drink, they simply waited for him.

The animals' behaviour was apparently highly unusual. "They would circle around, which is what they do with their young," says Walker.

The 21-mile North Channel crossing was the coldest of all and marked the completion of the Oceans Seven.

Walker is now a keen advocate for open water swimming, and runs his own swimming coaching business. "Open water swimming helps the immune system, it helps with depression," he says. "There are a lot of pools closing and it's very accessible. I've got my own aquarium when I'm swimming. It's an opportunity to see sea life and it has taken me to places I would never have been as a tourist. It's a discovery swim - you don't know what's coming."

For more information on the Global Open Water Swimming conference, go to www.vigourevents.com

adamwalkeroceans7.co.uk