Around Australia, hundreds of people are coming together to help a once-prized, but decimated and largely forgotten marine ecosystem. They’re busy restoring Australia’s native oyster and mussel reefs.

Mexico’s Cabo Pulmo is buzzing with sea life. Sharks, rays, sea turtles and humpback whales are just some of the species that rely on its vibrant coral reefs. It wasn’t always like this. In the 1990s, ...