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 under water, to make a Launch Star. One Chip in 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 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 gold shell with under water penguins.

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.