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."