Oceanworld

Oceanworld is a large, water-filled area of Mushroom World, which appears to have only one inhabited location, Happy Harbor, a port town.

In The Super Mario Bros. Super Show! episode "20,000 Koopas Under the Sea", Mario, Luigi, Toad and Princess Toadstool warp to Oceanworld in search of someone who can help them defeat King Koopa. Upon getting to Oceanworld though, Mario, Luigi, Toad and Princess Toadstool find it has problems of its own, as Happy Harbor is being terrorized by an apparent sea monster.

In the end, Mario and Luigi manage to save Happy Harbor from the sea monster, which is revealed to be a submarine known as the Koopilus, which belongs to Koopa Nemo.