Super Mario Galaxy/Bonefin Galaxy

Bonefin Galaxy is accessible from the Engine Room.

Star 1: Kingfin's Fearsome Waters
Head to the launch star to fight Kingfin. After landing in the water, find the green shell (it's hard to control your aim with the red shell) by swimming across the surface, then dive down to retrieve it. There is also a Power Up Mushroom near the red shell on the other end.

Once you get the shell and the mushroom, swim to the surface and travel the planet to find Kingfin. Dive, get near him, and take aim. Hopefully, you will hit him with the shell, and he will take damage. The second time, he will have little robotic fish around him. They won't do much more than Torpedo Teds: track you and try to hit you. Hit Kingfin five times in total to do him in and get the star.