Super Mario Galaxy/Beach Bowl Galaxy

Beach Bowl Galaxy is accessible from the Kitchen observation dome. You need 16 Stars to enter.

Star 1: Sunken Treasure
You need to collect five Star Chips underwater to make a Launch Star. One Chip is above a rock, one is on a ledge, one is in the clam, one is in a crate (spin to break the crate open), and one is by the Gringill (giant eel). Use the Launch Star to fly out of the water and to the upper part of the planet.

You need to climb to the highest point of the planet to find the Star. You can Ground Pound the switches to reveal temporary wooden structures to help you do this, or you can use Triple Jumps or High Jumps followed by a spin in the air to get there under your own steam. Spin Attack the crystal twice to release the Star.

Star 2: Passing the Swim Test
Find the gold shell then give it to the penguin instructor. You will find the gold shell with the underwater penguins swimming in a line. They will only show up if you talk to the penguin instructor first.

Star 3: The Secret Undersea Cavern
After landing, go underwater. A penguin will mention a secret crack in the wall. Grab a shell and throw it at the crack. The crack will burst and you will see a cave. Enter, then defeat the enemies and find the hidden switch. You will then be transported via a Launch Star to a secret level. Be mindful of your steps on this planet because the movements of the Tox Boxes are somewhat hard to follow. Finish the course and grab the star.

Star 4 (Fast Foe Comet): Fast Foes on the Cyclone Stone
This Star is available when a Prankster Comet is in this Galaxy.

Do the same as for the last section of Star 3, except with faster moving platforms and Thwomps. Luckily there are no Goombas this time.

Star 5 (Purple Comet): Beachcombing for Purple Coins
This Star is available when a Purple Comet is in this Galaxy.

You need to collect all 100 Purple Coins, with no time limit.