Unagi the Eel is a huge Unagi found in Super Mario 64 and its remake, Super Mario 64 DS. Mario or his friends can lure him out of his sunken ship home in Jolly Roger Bay by swimming near and provoking him to attack. After about attempted three bites, he will leave the ship, allowing Mario and his friends to enter it via the ship’s window. Once inside and he opens the chests in the correct order, the ship will surface, causing the water inside the ship to drain, allowing for Mario to get the Power Star.
Info from Mario Wiki