From the Super Mario Wiki, the Mario encyclopedia
The mechanical fish is an enemy in the Game Boy Advance game Wario Land 4. It looks like a yellow submarine in the shape of a fish, with a drill for a nose, a periscope, red eyes, and grey fins. The mechanical fish only appears in the boss battle against Catbat in the Sapphire Passage. They are found one at a time, swimming in the water channel in the boss room, swimming back and forth with the intention of harming Wario with its drill. The mechanical fish can only be defeated with an S-Ground pound.