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 that the ocean presumably surrounds is a post-apocalyptic setting. Shrimp are also seen swimming around buildings near the ocean floor.
This might also be the same ocean that was the Evil 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.
While Finn and Jake are playing by the sea, Finn realizes he is terrified of the ocean. The spectral Fear Feaster (voiced by Mark Hamill) taunts Finn about his fear, and Jake determines to help Finn overcome his fear of the ocean by traveling in it with a submarine.
The Ocean can be seen again in "Billy's Bucket List" where Finn tries to float on the ocean in order to complete Billy's bucket list. Finn finally lost his fear towards the ocean after his Grass Sword slashed Fear Feaster. Finn was fearlessly able to float on the ocean afterwards.