From the Super Mario Wiki
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 its 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.