Speaking ahead of the United Nations Ocean Conference, which begins in France on Monday, William said rising sea temperatures, plastic pollution and overfishing were putting pressure on fragile ecosystems and the people who depend on them.
"What once seemed an abundant resource is diminishing before our eyes," William, heir to the British throne, told the Blue Economy and Finance Forum in Monaco.
"Put simply: the ocean is under enormous threat but it can revive itself. But, only if together, we act now," he told the meeting of investors and policy makers.
This week's UN conference aims to get more countries to ratify a treaty on protecting ocean biodiversity which currently lacks sufficient signatories to come into force.
William addressed Sunday's gathering in his role as founder of the Earthshot Prize, launched by the prince in 2020 with the aim of making huge strides to tackle environmental problems within a decade.
On Saturday, William's office released a video of him talking to David Attenborough, one of the world's best-known nature broadcasters, about his latest documentary Ocean which examines the plight of the seas.
"The thing which I am appalled by, when I first saw the shots that were taken for this film are what we have done to the deep ocean floor," Attenborough told him.
"If you did anything remotely like it on land, everybody would be up in arms."
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