I have always loved the oceans. My father was a Navy man and one requirement he had for us growing up was that we had to live near a body of saltwater. I was raised listening to foghorns by night and being chased by horseshoe crabs by day.

The oceans are filled with so much life and variety - nearly all of it hidden from our sight. This makes the process of learning about the seas an endless series of surprises, a constant discovery of secrets. Like a lot of us, I always thought the oceans were infinite, vast and forgiving of what we were doing to them. They seemed somehow indestructible.

Now we know that's not true and these same features that make the oceans wonderful - their mystery and other-worldliness - have also worked to their disadvantage. Life beneath the surface is often out-of-sight and therefore out-of-mind. As a result, we tend to forget a rather important fact: we depend on the oceans for our survival regardless of where we live or what we eat. After all, our oceans generate most of our oxygen, regulate our climate, and directly provide a critical food source for much of our population. We cannot prosper unless the oceans prosper, too.

But as the oil disaster continues to ravage the Gulf of Mexico and the people who depend on it, we are being reminded daily of the often-forgotten value of these resources, and our responsibility to protect them. That's why I was filled with hope today when President Obama announced he is creating the first-ever comprehensive national policy  - like a Clean Air or Water Act - to protect our oceans. It is now clearer than ever that our country needs this to protect our oceans from the threats they face. If we had a policy like this in place before the Deepwater Horizon rig sunk - not only would we have better able to respond, an accident like this might not have happened there are at all.

This is the most significant action any U.S. President in history has ever taken for our seas. It will help make our oceans stronger and healthier, and help them fight off the myriad of threats they face today. It will help clean up the pollution that contaminates our beachwater, protect endangered species, keep the seafood we love on our plates, and make the oceans more resilient to the impacts of climate change.

My recent film, Avatar, took viewers to a magical place called Pandora, where the residents fight to save the natural world they depend on for survival from destruction at the hands of humans who have invaded their planet. You may not know that I also recently narrated a short documentary film called Acid Test, produced by the Natural Resources Defense Council, which looks at the fight to save our oceans right here on Earth. While we can see the impacts that the oil disaster is having on  the Gulf and its residents, Acid Test explores a less visual secret our seas have kept from scientists for too long - a phenomenon called ocean acidification - and what we must do to protect them from this new threat.

For decades, we've known about the link between carbon dioxide emissions and climate change. Only recently did scientists begin to realize that carbon dioxide emissions, about one quarter of which dissolve in the ocean and turn into acid, threaten dramatic changes from the bottom to the top of the food chain. Called ocean acidification, this process puts coral, shellfish and some plankton at direct risk and threatens repercussions to the fish, birds, dolphins and whales that depend on them for food.

If our oceans are to survive acidification, there are two critical steps we must take. First and foremost, we must cut our carbon emissions and transition to a clean energy economy routed in efficiency and renewable power. And second, we must make our oceans as healthy, and therefore resilient, as possible in the face of the coming impacts - President Obama has set us on that path today.

The creation of a national policy shows us there are solutions, and we can achieve them. It fills me with promise for a future of healthy oceans. And, in the face of the oil disaster and ocean acidification, it leaves me with hope that this generation, and those that follow, will still be able to share in the wonder of the sea for years to come.