He will swim for eight hours, jump on board the support boat to eat and sleep, and then enter the water again.
"I want as many people as possible to understand that each one of us has to take action, because the ocean is in peril right now, and if we don't change anything, it's going to be even worse in a few years", Lecomte said in an interview with GQ.
"I am not an Olympic swimmer, but I am an adventurer in the way that I push my limits", he added.
He will also wear a device to test levels of radioactive material from the Fukushima nuclear plant, which was hit by a tsunami in 2011.
"To do the physical aspect of it, yea, it is hard, but what is much more hard is to be in that very hostile environment, and the mind has to be super strong", he said.
"When you don't have anything to occupy your mind, it goes into kind of a spiral, and that's when trouble starts", he said.
"I am a little bit like a tiger in a cage, ready to go".
Men's quarter-finals rained out, to resume Thursday
In the other men's quarter-final, Juan Martin del Potro and Marin Cilic were locked at 5-5 in a first-set tiebreaker. That proved costly as Nadal broke and then repeated the trick to move into a 5-3, 30-15 lead.
He plans to swim eight hours at a time, eating 8,000 calories a day and resting each evening on a support boat.
But Lecomte will actually be in the water, potential shark bait.
He faces dangers including sharks, storms, swarms of jellyfish, and extremely low water temperatures. If he makes it, he'll be the first to swim the length Pacific.
Scientific teams accompanying Lecomte will collect more than 1,000 water samples and study plastic pollution, mammal migration and the effect of extreme endurance events on the human body.
"What is going to be hard is every morning going back in the water (because) you hit a wall, normally after 4-6 hours", he said of the mental challenge.
"It didn't happen very soon after the Atlantic (swim) because I got married, I had children, so I put that aside".
"I will remember a family birthday for example and the trick is to engage all your senses-try to remember the wind on your skin, how the sun felt, the smells".