Green Star #1
From where you start, walk down the ramp into the water, and make your way to the first Green Star you see. Be careful to avoid the Rammerhead—Captain Toad is not a very fast sprinter, and he can’t swim, which only slows him down even more.
Green Stars #2 and #3
Now, get to the other side of the Clear Pipe, and take the passage in the corner to get to the other side of the level. From there, you can take the Clear Pipe to reach another one of the Green Stars. Next, take the Clear Pipe up to the other exit you didn’t use before, and turn the camera back around to the “front” of the level. Follow the wooden planks back round to the back of the level, and fall onto the raised grass platform.
You can now follow the grass walkway to reach the third Green Star. Use the Clear Pipe just beyond the third star to reach the upper portion of the level.
Green Stars #4 and #5
Fall onto the P Switch to raise the water level. The rising water will brings the fourth Green Star up to the ledge with the P Switch. This also raises the whole wooden walkway up to your current position.
Reaching the last Green Star is now a matter of following the newly raised wooden walkway around the level until you can reach the ramp that takes you to the star. This is complicated by the Rammerhead—It’s still around, and it can ruin your day if you mistime the run across the wooden walkway.