Check out this guide on The Secret Undersea Cavern (Beach Bowl galaxy) Hidden Star in Super Mario Galaxy Switch (All Stars 3D). Includes how to unlock the Hidden Star, how to get, and location!
Wall Jumping Up Waterfalls Walkthrough
Wall Jumping Up Waterfalls - How To Unlock
Head To The The Secret Undersea Cavern

You will only get the Hidden Star by going into the Secret Undersea Cavern in the Beach Bowl Galaxy.
Grab A Shell From Under The Water

Grab any of the Green Shells from under the water and bring it back to land.
Throw It At The Chest

You will need to throw the Green Shell directly at the chest close to the cliff to get the Star Portal to where the Hidden Star is.
Wall Jumping Up Waterfalls - How To Get Hidden Star
Wall Jumping Up Waterfalls Walkthrough Procedure Chart
No | Procedure |
---|---|
1 | Lure one of the blue birds under the giant coin |
2 | Use the bird to jump up and get the coin. This will unlock an Ice Flower |
3 | Grab the Ice Flower and proceed to walk on the water |
4 | Jump kick on the waterfalls to get to the top of the floating platform |
5 | Use another Ice Flower and create a path for the Blue enemy to follow you |
7 | Lure it to the small circular platform beneath the star |
8 | Jump up to get the star! |
1. Lure Blue Cataquack Below The ? Coin

In order to reach the ? coin above you, you need to bring blue Cataquack just below the ? coin so that it can launch Mario up in the air.
3. ~ 4. Get To The Top Of The Platform - Tips
Jump Kick The Wall With Ice Mario

Ice Mario can climb up the waterfall by using jump kick. Stand in between two waterfalls and use jump kick to climb up.
Ice Flower Power Can Run Out

If you don't move fast enough, you will lose your Ice Flower powers as you attempt to climb to the top. If this happens, you can grab another Ice Flower and try again.
5. Create A Path On The Water - Tips
Create A Path For Cataquack

Ice Mario can create a frozen path for Cataquack. Move it just below the star and use Cataquack to reach the star.
Spikes Will Not Remove Ice Power

If you get hit by the spikes on the water, you won't lose your Ice Flower power. However, you will get stunned for a bit.