In this level the water will rise and fall, so you will need to follow the water sometimes or have to wait in order to continue. At the beginning, go underneath the wall on the right side and swim up (you can also wall jump, if you’re a bit impatient) until you reach some spiked blocks. When you can, jump into the higher body of water and go to the top left corner to find the Stamp. In the same section, there will be some ?-Blocks in the lower right corner, one of which has a power-up (Super Bell). Continue swimming up, above where you grabbed the power-up, until you reach dry land.
You will need the Cat Suit here, so hopefully you have one. You should notice an opening up and to the left of right where you jumped out of the water. Climb up there to discover a hidden alcove, with Green Star #1. Continue to the right, touching the Checkpoint Flag, and board Plessie. There’s nothing in this section, other than some coins and Madpoles, so there’s no need to search. At the end, climb the wall to the left or right of the opening to reach a ledge, then climb more to find Green Star #2.
Jump down and cross the donut blocks, swimming up to the next ledge, where you’ll find a power-up in one of the ?-Blocks. The next section will have you swimming in little pockets of water, changing to new ones, while dodging the moving blocks with spikes. When you get to the end, don’t go in the pipe, instead climbing the wall above it to find a cloud cannon. Race to the end of this bonus area to collect Green Star #3, then you’ll be deposited in the Goal Pole area. Use the moving water to get to the top, allowing easier access to the top of the pole.