Even if you swear by the Discovery Channel, you might find something new in “Sharkwater,” a personal documentary by Rob Stewart. Call this movie an environmental swashbuckler.

It’s about sharks, those feared predators of the deep blue sea. Stewart, a Canadian underwater photographer and rabid shark buff, is here to tell us that we shouldn’t be afraid.

Stewart spends the first half-hour of the film showing some great shark footage (including a swirl of hammerheads near the Galapagos) and providing facts, rather than myths. He makes a good case that sharks are key to the entire ocean ecosystem, and indeed that sharks, having been around for at least 150 million years, have largely determined what the ocean’s population is.

He also points out that elephants and crocodiles kill more people each year than sharks do. To prove his point, we see him swimming around and petting various sharks. “If they wanted to eat us,” he says, “they would.”

Then the film shifts into Stewart’s passionate, genuinely disturbing account of the way the international trade in shark fin is seriously damaging the world’s population of sharks — and thus damaging the balance of life in the sea. Shark fin is a prestige meal in Asia, as well as supposedly having medicinal benefits.

This is where the swashbuckling comes in. Stewart signs on with Paul Watson, a captain and co-founder of Greenpeace, whose response to illegal whalers is sometimes violent. Stewart and Watson confront some shark poachers off Costa Rica, only to find themselves arrested in port.

Stewart proceeds to document the way Costa Rica’s government has protected the lucrative shark fin business. He and Watson escape the place in a boat race with the authorities. The dangers in this movie come from man, not shark.

The filmmaker-star then documents the way even the Galapagos Islands, that sanctuary of natural life, relaxed its laws against shark-hunting. The facts and footage here (there’s plenty of video evidence of shark-finning) are persuasive, although at a certain point Stewart seems to run out of luck when it comes to finding drama.

Then he has only his own movie-star looks to fall back on. This might make the film’s environmental message more popular than it would have been otherwise. Nobody wants to see Michael Moore free-diving in a Speedo, for instance.

Still, this movie has an urgent message, and Stewart brings it to life. There are a lot of other things to worry about in the world, but “Sharkwater” makes a good case that sharks are right up there in importance. Sharks were around a long time before mankind, and they might outlive us yet.