Jeremy McClure has lived in darkness for half his life, but the blind four-time Paralympian swimmer has never let that hold him back.

The 30-year-old has been preparing to take on his biggest challenge yet — attempting to become the third person ever to complete a triple Rottnest crossing.

The almost 60 kilometre open ocean swim will mean 22 hours in the water battling potential swell and strong currents, as well as the heat of the day, the dark of the night and the threat of sharks.

But for Mr McClure the journey is simply the next step in what has been a lifetime of setting his own goals and challenges.

"I'm essentially adding one more leg to the double crossing, which I did last year from Cottesloe to Rotto and back," he said.

"People like my wife couldn't think me more crazy with things like this but it's the type of person I am.

"I am extremely happy when I can accomplish something and I just like to then aim for something else to keep busy."

Swimming away the pain

Despite being born with a hereditary optic nerve disease, Mr McClure was unaffected by his condition for most of his childhood.

"When I turned 15 — over about six to eight weeks — I lost 98 per cent of my vision ... so I've got about two per cent," he said.

"It was a very sudden onset … no-one was really aware it was going to happen because, even though it runs in the family, the last person alive that had the condition was many years ago."

Jeremy McClure will be tethered to another swimmer throughout the triple Rottnest crossing he is attempting. ( ABC News: Briana Shepherd )

Despite having previously disliked swimming, it became a blessing during that time.

"I just did it because my parents made me and now I thank them for that because it was something that I knew I could do, just not very well," he said.

"I couldn't play team sports anymore as it was too dangerous so I thought, 'oh well, in a couple of years there's the Paralympics in Athens [2004], let's give that a crack'.

"I changed my mindset and actually focused on something to distract myself from being a bit sad, as you would be when something severe like that happens."

'There's something different in the open water'

Mr McClure will have a group of 11 swimmers to guide him and look out for sharks. ( ABC News: Briana Shepherd )

Mr McClure will set off from Rottnest for the mammoth swim on Wednesday evening, with the intention of getting the worst part — the swim at night — over and done with first.

He will have a team of 33 people around him, including seven kayakers, eight skippers in four boats, a time keeper, his wife and a shark scientist.

He will also have a group of 11 swimmers who will share support duties — either being tethered to him and essentially becoming his eyes in the open ocean, or swimming nearby with a Shark Shield device strapped to their ankle.

His parents will meet him at each end, ready to lather him up with sunscreen and Vaseline while he takes no more than a ten-minute standing break in the water.

Mr McClure, a backstroke swimmer in the pool, said after months of preparation he was as ready as he would ever be to make the swim.

"There's something different in the open water, that little bit more freedom, not just going up and down a pool," he said.

"And it's something where I'm involved with other people, so it's almost like a team environment where I have to rely on people to guide me.

"It makes it more fun, because everyone's achieving it and I couldn't do it without them and their support so it makes it all the better."