From the Super Mario Wiki
Pirate Waters is a large body of water located in Pirate World; Pirate Waters was seen in The Super Mario Bros. Super Show! episode "Pirates of Koopa" According to Mario, Pirate Waters is one of the most treacherous seas in the world, with Captain Clump and Blackbeard Koopa apparently being the only sailors brave enough to sail it. Near Pirate Waters is Pirate's Port, a port apparently inhabited by a variety of pirates and scoundrels. The only fish shown to inhabit Pirate Waters are Trouters, which readily attempt to devour anything that falls into the sea.