The ocean is featured in the episode "Ocean of Fear." It is the main setting in this episode, when Jake helps Finn to conquer his fear of the ocean. There is an underwater city here, as well as a lot of debris, including a tank and an aircraft carrier. These might be clues suggesting that the Land of Ooo is a post-apocalyptic setting. Shrimp are also seen swimming around buildings near the ocean floor.
This might also be the same ocean featured in the flashback of Billy's Song, which was played in the episode "His Hero." It was seen when Billy ran up to it and slashed at a wave with his sword.