This galaxy isn't that big with one main mission. The main planet of this galaxy is the Buoy Base which is also the starting planet. It consists of a large pool of water and a tall tower with various obstacles including Topmen, Thwomps and Amps. The idea is to collect the blue Star Chips to go to the other planet with a Pull Star. A secret cage containing a Warp Pipe is submerged with Bill Blasters firing Torpedo Teds at Mario. Going through the Warp Pipe leads to the other side of the planet with more Bill Blasters and a cage containing a Green Power Star
The other planet is a giant sphere filled with water above the Buoy Base. It has a screw on it that Mario spins to open the planet and retrieve a Power Star. It also has two pulse-wave generating machines, one of which is actually the screw. It is remarkably small in comparison to the Buoy Base and is similar to a Poké Ball.
The Floating Fortress
The Secret of Buoy Base (Green Star)
Buoy Base has a unused low poly version that shows much more islands and content under the level. The low poly version indicates it was originally going to be part of a much larger galaxy.