This page offers a walkthrough of the Hidden Star found within The Secret Undersea Cavern level in Beach Bowl Galaxy.
Smash the Chest
First off, you’ll need to grab a shell from under the water. Then, walk up to the shore on the left of the pier that you started on. You’ll need to walk around and behind the waterfall and then you need to jump down onto a lower platform. There you will find a chest that you can smash open by using the shell you’ve brought with you. It will reveal a Launch Star that will take you to a secret area.
Lure the Cataquack
When you land in the new area, you’ll notice a Golden ? Coin high in the sky. You’ll need to lure one of the Cataquacks over to underneath the coin. When you jump on them they give you a big boost so you can use that to grab the coin. Be careful though, if the cataquack falls into the water, you’ll need to begin luring it again. When you’ve grabbed the coin, a platform beneath will rise slightly and an Ice Flower will spawn.
When you use an Ice Flower Mario becomes Ice Mario! Whilst in the Ice Mario form, you can walk on water and create temporary ice hexes. The Ice powers are only temporary, so you’ll have to quickly run over to the two waterfalls on the far end of the lake (not the ones to the left of the ice flower spawn point). Then wall jump between the waterfalls to reach the higher platform. When you get to the platform quickly use the sling star to get to the next platform. Wall jump between those two waterfalls as well until you reach the higher platform. On this platform there is only one waterfall. You need to jump off that waterfall and into the sling star to get to the final area of this level.
Reach the Power Star
In this next area, you’ll need to guide a Cataquack across the water and over to the Power Star so you can jump up to it. There will be an Ice Flower at the beginning that you can use to create a path for the Cataquack. The easiest way to create a safe path is to head to the platform on the right first with the cataquack following, and then head over to underneath the Power Star. Watch out for the big ice chunks that will smash your path!