Dr. Sylvia Earle is the world’s greatest living ocean explorer. Now nearing 80, she has spent more than 7,000 hours underwater and has witnessed first-hand the damage done to the seas. She is deeply worried. As she says, “No blue, no green. No ocean, no life—and, therefore, no us.” Vital fisheries are depleted; half of all coral reefs are dead or dying; huge “dead zones” have emerged; shorelines and wetlands have been devastated. As this remarkable documentary—released Aug. 15 on Netflix—shows, human predation and pollution are the causes. At once a portrait of Earle, whose love of the oceans is matched only by her dignity and grace, the film is also an underwater survey rich in gorgeous images of life in those ocean areas that remain healthy. But it also reveals the awful ugliness of dead oceans; the contrast is depressing. The goal of Mission Blue, the organization she founded, is to expand what she calls the “hope spots”: ocean areas set aside and protected, much as national parks are protected on land. Currently 3 percent of ocean waters are safe; she wants to get that figure to 20 percent. Now’s the time, she says. Fifty years from now will be too late.