Sunken Ghost Ship

A wrecked ship that serves as a gate to the Valley of Bowser, after defeating Wendy O. Koopa Mario has to swim through this ship in order to make the doorway to World 7 come out of the sea. On board the ship are some Bill Blasters, Floating Mines, Koopas, and a massive swarm of Disappearing Boo Buddies.

It's one of the Koopa Kids Air Ships from Super Mario Bros. 3. Probably Larry Koopa's because his castle is in the Valley of Bowser.