Endless Ocean places the player in the role of a scuba diver exploring the Manaurai sea in search of sea life and sunken treasure under the guidance of an assistant named Catherine Sunday. In the sea, they will encounter a number of marine species ranging from smaller fish and penguins to massive whale sharks, manta rays and humpback whales. The range of marine wildlife in the game is extensive and includes many common and rare species. The player will also encounter dolphins and other cetaceans that can be trained to perform certain tricks and become companions on dives. Species such as sharks are also present; however, they pose no threat to the player. The player also has access to a large aquarium that they can populate with species they have identified. The sea's various locations providing a means for the player to experience general diving, cave diving, deep-water trench exploration, wreck diving, and other activities that might not otherwise be possible in a single real world setting.