Leonardo DiCaprio has explored the deep seas of Mozambique, Thailand and beyond; he's swam with 15 species of sharks and narrowly avoided being attacked by one.

And no, it wasn't for one of the A-list actor's movie roles. It's just what he does for fun.

DiCaprio appeared at the State Department in Washington, D.C., Tuesday to talk about an issue near and dear to his heart: the protection of the world's oceans, which, he warned, are in danger of being destroyed by climate change, illegal fishing and other human activities.

"I'm standing here today as a concerned citizen of this planet who believes that this is the most important issue of our time," DiCaprio said. "If we don't do something to save our oceans now, it won't be just the sharks and the dolphins that will suffer; it will be all of us including our children and our grandchildren."

DiCaprio's appearance was part of Secretary of State John Kerry's two-day "Our Oceans" conference, which focused on framing the world's waterways as a global security and economic priority. Just before DiCaprio spoke, President Obama announced executive actions geared towards combating black market fishing and protecting world-class marine areas including parts of the Pacific Ocean.

And DiCaprio added his own contribution to the effort, announcing that his eponymous foundation would be making a $7 million pledge to ocean conservation projects over the next two years, in addition to an earlier pledge of $3 million towards marine wildlife preservation.

DiCaprio's love of the ocean began as a child, he explained, telling the audience that he dreamed of being a marine biologist, before he got into acting.

"As a kid, I always had a fascination with the ocean and its wildlife. In fact, the first philanthropic dollar I ever contributed was to save the wild manatee in Florida," he said.

And that passion continued into adulthood, as he became an avid diver.

"Since my very first dive in the Great Barrier Reef in Australia 20 years ago to the dive I got to do in the very same location just two years ago, I've witnessed environmental devastation firsthand," he said.

He lamented how, during a recent trip to a marine reserve on the shores of Costa Rica, he watched as illegal fishing vessels breached an off-limits area just a mile offshore.

I've learned that with each passing day, so many of our aquatic species are in jeopardy - not because of nature's unpredictability, but because of human activity," DiCaprio said.

The star finished his speech with a message for leaders worldwide to make ocean viability a priority.

"I stand here today to challenge all of you to step up, to utilize your positions of authority, to ensure the health of the oceans that are so vital to people's lives all around the world," he said.

"My ask is straightforward: Step up."