As of Wednesday, Aug. 6, Adam Walker became the first Brit to successfully complete the seven hardest and most dangerous ocean swims in the world, encountering all sorts of marine life and battling sickness along the way.

The Nottinghamshire swimmer started the Ocean's 7 Challenge - which only four people have completed - in 2008 with a swim across the English Channel.

He has since conquered seas in Gibraltar, Hawaii, New Zealand, the United States and Japan.

His seventh and final swim was across the North Channel, spanning from Northern Ireland to the west coast of Scotland. He completed the grueling 21-mile (33-kilometer) journey last Wednesday.

"It's just not real really; seven years ago I wasn't swimming at all," Walker told BBC News of his achievement.

Walker told Nature World News (NWN) that he is both ecstatic and relieved to complete his seven ocean swims, during which he had been stung by jellyfish, encountered sharks and been protected by dolphins.

Swimming Among Monsters

During this last leg of the series, Walker swam amongst Lions Mane jellyfish, which are considered the most potent jellyfish in the world. They are also the largest - running up to 6.6 feet (2 meters) in diameter with tentacles reaching 49 feet (15 meters) long, according to National Geographic. Usually brief contact with the jellyfish causes temporary pain and local redness, but severe contact (for instance, accidentally swimming straight into one) can be deadly.

And Walker encountered numerous of these Lion's Mane fish - so named for their long tentacles - during his swim through the North Channel.

"At one stage I had so many lions mane jellies surrounding me the only exit I had was to swim backwards," he told NWN.

What the endurance swimmer also didn't realize was that even when these marine creatures shed their tentacles, they still have a nasty bite, and so he was subsequently stung several times.

Though, Walker says this does not compare to when he was stung by a Portuguese man o' war (Physalia physalis) off the Hawaiian coast in the Molokai Straits. During the 26-mile challenge, Walker was in extreme pain following the attack, yet he still had to swim another 3.5 hours in agony to finish his 17-hour swim. He experienced paralysis 13.5 hours later.

"I was stung after 14 hours and I can only describe it like someone had slashed me repeatedly across the front of my stomach and down the side," he told BBC News back in 2012.

"I was shouting in agony for around two minutes, holding my stomach and realized I had a 5-inch (12-cm) tentacle stuck to my stomach, which I ripped off."

These venomous jellyfish, with tentacles typically 30 feet (10 meters) in length, are covered in venom-filled nematocysts used to paralyze and kill fish and other small creatures. For humans, a man-of-war sting is excruciatingly painful, but rarely deadly, according to National Geographic.

Throughout this competition, the 35-year-old has been raising money for marine charities, including the Whale and Dolphin Conservation Society.

Walker is probably most recognized from his YouTube video that shows the British swimmer surrounded by a pod of 10 dolphins, protecting him from a shark lurking below across the Cook Strait in New Zealand. The video has since received over four million views.

"I'd like to think they were protecting me and guiding me home," Walker wrote on his Facebook page. "This swim will stay with me forever."

As well as seeing numerous dolphins during his swims, Walker has also swam in close proximity to various sharks.

Tiger sharks, for instance, the leader in human attacks in Hawaii, were in ocean water in the Molokai Straits with the long-distance swimmer. According to the NOAA, these are one of the larger shark species, believed to exceed 18 feet and 2,000 pounds. Aside from the Pacific coast, tiger sharks (Galeocerdo cuvier) can be found off the Atlantic coast, from Massachusetts to the Gulf of Mexico and the Caribbean Sea.

"I have never seen a shark before on any of my swims, but this one, which was at least two meters long, followed us for a few minutes at a depth of about 15-20 feet," Walker told BBC of a tiger shark encounter.

"I do remember thinking, wouldn't it be just my luck if a shark took me 100 yards from the shore," he added.

Walker was also on the lookout for great white sharks, as well. These feared predators can now be seen off Cape Cod, Mass. and are boosting local tourism due to curious beachgoers.

The Smithsonian Institution reports not just of their size and ferocious bite (they have 300 teeth total), but of their speed also - they can swim up to 35 mph.

Walker has also swam close to a pilot whale in the Straits of Gibraltar. Pilot whales are actually a type of dolphin, and is second to only the killer whale in size. They feed primarily on squid, although it's known to eat octopus, cuttlefish, herring and other small fish, reports the American Cetacean Society.

Pilot whales have recently been getting a lot of media attention due to the dolphin hunts, or "dolphin grind" occurring in the Faroe Islands. Even celebrities are taking part in the fight to protect these marine animals from mass slaughter. Every year, from June-October, Denmark fishers corral numerous pilot whales together and brutally kill them with hooks for their meat.

"Having swam near to them in Ireland and hear them calling to each other on Gibraltar straits further enhanced my respect and passion for them," Walker told NWN. "I cannot understand why human beings would want to hunt them, they are magnificent and whaling makes me so sad."

Nature and Health

Walker also had other encounters with nature, so to speak, during his seven ocean swims. He said the sea temperature on the latest leg across the North Channel was the coldest of the lot, and he faced severe hyperthermia.

According to WebMD, hypothermia sets in when body temperature drops dangerously low, usually caused by prolonged exposure to cold temperatures. Waters in the North Channel run at about 0.5-14 degrees Celsius (50-54 Fahrenheit). Normal body temperature is 98.6 F on average, but drops below 95 F with hypothermia. In severe hypothermia, core body temperature drops to 86 F or lower.

Normally, activity of the heart and liver produce most of our body heat, but as core body temperature cools, these organs produce less heat. They eventually shut down to preserve heat and protect the brain. Symptoms include fatigue and confusion, which Walker experienced.

"I couldn't feel my feet, arms, hands and then face and started to get paranoid," he told NWN. "It was a scary situation."

Walker now runs swim camps through his company "Ocean Walker" to educate people on the safety measures on how to warm up quickly but safely.

He also tells NWN that he will continue to do swims to raise awareness and money for whale and dolphin conservation.

"The sport has given me so much so I want to encourage others to follow their dreams," he added.

You can learn more about Walker and his ocean swims on his website, www.oceanwalkeruk.com, and follow him on Twitter (adamsoceans7).

[Credit: YouTube/KITV]