Oceanworld

The Ocean World 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 warped to Ocean World in search of someone who could help them defeat King Koopa. Upon getting to Ocean World though, Mario, Luigi, Toad and Princess Toadstool found it had problems of it's own, Happy Harbor was being terrorized by an apparent sea monster.

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