A FRENCHMAN embarked on a 9,000-kilometre journey from Japan to the US west coast, hoping to become the first person to swim across the Pacific Ocean.

Ben Lecomte dived into the water off the coast of Choshi, east of Tokyo, on Tuesday morning, planning to reach San Francisco in six months' time in an attempt to raise awareness about the ocean's health.

A live-stream of his departure was posted on Facebook.

Lecomte is planning to swim for eight hours a day, aiming to advance 65 km each day, accompanied by a support sailing boat and his six crew members.

"I'm not an Olympic swimmer, but I am an adventurer in the way that I push my limits," said Lecomte, who became the first person to swim across the Atlantic in 1998, in just 73 days.

The team, working with 12 scientific institutions, including NASA and Woods Hole Oceanographic Institution, will conduct oceanic and medical research throughout the journey.

"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.