Ocean Small Turtle
From the Super Mario Wiki
Ocean Small Turtles are a pair of mechanic transports found on a planet in Dreadnought Galaxy in Super Mario Galaxy. When Mario floods the planet by luring a Bullet Bill to break a vault, these two turtles (which were before, semi-buried in the floor) come out and start moving across the surface of the water. They are only useful for getting some Star Bits, and serving as a platform to climb back onto the main body of the area.