Adam Walker, 33 of Nottingham, England wants to become the first swimmer to complete the Ocean's Seven, a series of seven open-ocean swims across some of the most challenging channels in the world.
He's already swum the English Channel and the Strait of Gibraltar.
Next on his list was the Kaiwi Channel from Molokai to Oahu, known as one of the toughest channel crossings in the world because of the massive swells, abundant marine life and strong currents.
Thursday morning at 5:55 Walker stepped off La'au Point on Molokai and began swimming toward Oahu.
He thought the 26-mile channel crossing would take 12 to 13 hours. Little did he know his third challenge of the Ocean's Seven would turn into a seventeen-hour ordeal.
It was 10:55 p.m. when Walker finally stepped on the beach at Diamond Head on Oahu.
"I don't know what to say really, it was torture," Walker told KITV4. "I just kept thinking when I was swimming towards the end, the thought of waking up tomorrow not doing it."
The first obstacle facing Walker was the lack of proper rest.
After a 20-hour flight that brought him to Oahu on Tuesday, the Brit was in the water just a couple of days later.
"He's had no rest," said Chris Sheppard, Walker's friend and training partner back in England. "He's had two disturbed nights of sleep; maybe four hours each night."
During the first six hours of the Kaiwi crossing, Walker was on pace to break Darren Miller's record of 12 hours and 12 minutes.
He had already been stung in the face by jellyfish, but kept on going.
During the swim Sheppard got in the water to encourage his friend.
However, he never told Walker about a six-foot shark that was swimming about 20 feet below them for nearly a half hour.
"He could have swam off and he didn't," said Sheppard. "He just stayed with us and stayed with us and stayed with us."
Although Walker was wearing an electronic device around his ankle to repel sharks, he said the large animal below him was the least of his worries. He was more concerned with his growing fatigue.
As Walker came within three miles of Sandy Beach on the southeast tip of Oahu, the current began to change and he was going nowhere.
"It started getting stronger, and it was taking us away from the beach," said Sheppard, who was on Walker's guide boat. "We thought we'd get in in two hours, and it took us about six."
Adding to Walker's misery, he was again stung by jellyfish, causing him to vomit and scream out in agony.
"That is the closest I've ever been to giving up on a swim," he said, "but I just kept thinking, no, no while you can still move, keep going."
"It makes you stand back and think, 'That bloke is amazing,'" added Sheppard, who stands in awe of his friend's tenacity.
When it was all over, Walker couldn't even contemplate his next challenge of the Ocean's Seven, the Catalina Channel from Catalina Island to the Southern California coast.
However, after a day of rest, the Brit appeared more resolute than ever to scratch number four off his list.
"Hopefully I'll go down in history as one of the best open water swimmers there's been," he said. "That would be fantastic."
Walker plans on challenging the Catalina Channel sometime next year.
As he works down the list of the Ocean's Seven, he's also raising funds for two charities: SportsAid and the Whale and Dolphin Conservation Society.
"I would hope that I went through all that in order to raise money for these fantastic charities, to make it all worthwhile," he said.
Walker's trip to Oahu was sponsored by the Equuas Hotel and Marina Tower in Waikiki, the makers of the Electronic Shark Defense System, and the Indesit Company, which employs Walker in England.
The Ocean's Seven Challenge is as follows, but there is no order of difficulty:
1. North Channel, between Ireland and Scotland
2. Cook Strait, between the North and South Islands of New Zealand
3. Molokai Channel, between Oahu and Molokai Islands in Hawaii
4. English Channel, between England and France
5. Catalina Channel, between Catalina Island and the Southern California mainland
6. Tsugaru Channel, between the islands of Honshu and Hokkaido in Japan
7. Strait of Gibraltar, between Europe and Africa