From the Super Mario Wiki, the Mario encyclopedia
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Happy Harbor is a port town located in the Oceanworld; like most ports, Happy Harbor has a variety of ships located in the waters near. It is also specialized in shipping goods.
In The Super Mario Bros. Super Show! episode "20,000 Koopas Under the Sea", King Koopa, wanting to conquer Happy Harbor, begins to attack it with his submarine, the Koopilus, which the Mushroom inhabitants of Happy Harbor believe is a sea monster. King Koopa promises to get rid of the attacking sea monster if the residents of Happy Harbor make him their ruler, though this plan of King Koopa's is spoiled by Mario, Luigi, Toad, Princess Toadstool and Captain Abidab, who reveal King Koopa's evil nature to the inhabitants of the town.
Having captured Mario, Luigi and Princess Toadstool, who were hunting what they believed to be a sea monster with Toad and Captain Abidab, Koopa Nemo reveals to them that, after they ruined his previous plan, instead of taking over Happy Harbor, he will simply destroy it with the Koopilus. This plan of Koopa Nemo's is eventually foiled by Super Mario and Super Luigi, who destroy the submarine so it can no longer menace Happy Harbor.