From the Super Mario Wiki, the Mario encyclopedia
In The Super Mario Bros. Super Show! episode "20,000 Koopas Under the Sea", King Koopa holds a rally claiming he can get rid of a sea monster that is terrorizing Happy Harbor, but is confronted by Mario and Princess Toadstool. This leads Captain Abidab to show up and claim that she can get rid of the sea monster, which King Koopa scoffs at before leaving with his two Bloober minions. Everyone else runs away, and Captain Abidab recruits Mario, Luigi, Toad and Princess Toadstool as her crew.
Later, Captain Abidab introduces Mario, Luigi, Toad and Princess Toadstool to her ship, the Peapod. She begins to question Mario, Luigi and Toad's effectiveness as her crew, as the three of them prove highly incompetent as sailors; Captain Abidab is reassured of their effectiveness by Princess Toadstool, who shows some doubt at her own words.
When the search for the sea monster proves fruitless after a while, the captain decides to expand the search and go underwater. After putting on a diving helmet and giving Mario a plunger-loaded harpoon gun, Captain Abidab dives, with Mario, Luigi, Toad and Princess Toadstool following her. The team is soon ambushed by the sea monster, which Mario manages to hit with a plunger launched from his harpoon gun. Grabbing on to the rope dangling from this plunger, Mario, Luigi and Princess Toadstool are pulled by the sea monster and devoured by it, much to Toad and Abidab's horror. When the sea monster begins to approach them, the two manage to escape its sight by hiding in an underwater pit. Captain Abidab and Toad then follow it to the surface, where they see it bash into the Peapod before submerging underwater.
Later on the ship, Toad and the captain find a message in a bottle from Mario and the others, setting sail for a shortcut to Happy Harbor. By the time they arrive at Happy Harbor, Koopa Nemo has already begun to attack the ships with his Koopilus submarine, the supposed sea monster. Using a makeshift catapult, Captain Abidab launches Toad into the Koopilus's mouth to help his friends.
After the Koopilus is destroyed and Koopa Nemo is defeated, Captain Abidab and others cheer for Mario and Luigi for them having saved Happy Harbor.