Bonefin Galaxy is accessible from the Engine Room observation dome. You need 55 Stars to enter, one of which must be from completing Giant Eel Outbreak in Drip Drop Galaxy.
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.
Once you get the shell, 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 Bullet Bills: track you and try to hit you. Hit Kingfin five times in total to defeat him in and get the Star.