Several Earthshot winners and past finalists were in the audience.
Enric Sala, of the National Geographic Society’s Pristine Seas project, was a finalist in 2021 and has pioneered work to protect marine life.
He is also part of the team that has produced Sir David Attenborough’s new film, Oceans, which Prince William described as “the most compelling argument for immediate action I have ever seen”.
“Watching human activity reduce beautiful sea forests to barren deserts at the base of our oceans is heart-breaking,” the Prince said.
“For many, it is an urgent wake up call to just what is going on in our oceans. But it can no longer be a matter of ‘out of sight, out of mind’.”
He ended his speech saying action was needed for future generations and quoted Sir David.
“If we save the sea, we save our world.”
The Prince interviewed Sir David at the premiere of Oceans last month, with the film described by its producer as “the greatest message [Sir David] has ever told”.
Kensington Palace described the speech as a “landmark intervention” by Prince William, using his platform to generate change and bring in investments to scale up ocean solutions.
While in southern France, the Prince met President Chavez of Costa Rica, France’s President Macron and Prince Albert of Monaco – a supporter of many oceans projects and a key player at the forum.
Prince William will also attend a closed session, held in private, with ocean experts and investors.
Additional reporting by Adam Hale.