The Queen Anne's Revenge, which was the pirate Blackbeard's ship, off the coast of North Carolina; manta rays that feed off the Kona coast in Hawaii, including one that has been coming to the feeding site since 1979.
A visit to the Mystic Aquarium in Connecticut spotlights beluga whales, including how they're trained and cared for. Also: a 400-foot-deep blue hole located on Andros Island in the Bahamas; a playful pod of spinner dolphins off the Hawaiian coast.
The Season 4 premiere travels to Manuk, a small volcanic island in Indonesia, to learn why its waters are teeming with thousands of venomous sea snakes. Also: a visit to a remote atoll in the Maldeves where manta rays gather for a few weeks every year.