The water level rises and falls throughout this level, sometimes with separate sections of water doing their own thing. You don’t have to necessarily wait for the water to come to you if you use wall jumps.
Stamp
You can get the Stamp pretty early on. Wait for the water to rise up and allow you to avoid the Spike Blocks under the Stamp. While you’re in this area, don’t miss picking up the Cat Power-Up from a nearby ? Block. You’ll need it.
Green Star #1
If you scale the wall on the left of the passageway leading up from the Stamp room, you can find a hidden alcove with the first Green Star.
Plessie waits for you near the Checkpoint Flag, but it’s a short ride to the next section of the level.
Green Star #2
The second Green Star is atop a pair of Donut Blocks atop the wall Plessie drops you off at. You’ll need the Cat Power-Up to reach it.
This next section can be pretty rough! Using the moving water cubes, you’ve got to navigate past the moving sets of Spike Blocks. Don’t get caught underneath a Spike Blocks that’s moving downwards because you can get forced out of the water and into the bottomless pit.
Green Star #3
Ignore the pipe you find just past the moving water cubes for the moment. Climb up the wall it’s on to find a Cloud Cannon that takes you to a bonus area where the third Green Star awaits. Watch out for the Para-Biddybuds in this area!