As the first real underwater level of the game, it might be a good idea to bring along a Fire Flower, as that power-up is very useful in these kinds of levels. Cross the water, and the Goombas in the swim rings, and jump on the invisible Coin Coffer in the bushes to the right of the pipe for a few quick coins. Take the pipe to reach the next area, where you can climb up the wall with the Cat Suit, to a tree that has a Super Mushroom. Drop back down and go into the water, where you’ll find a Roulette Block, with the chance to grab a power-up. As you continue right, you will run into some Cheep Cheeps, which do nothing but swim back and forth.
The clear pipes you see embedded in the floor and wall are hiding nothing here, so ignore them. The Porcupuffers are similar to the Cheep Cheeps, but they are bigger and spiky, so you may want to get rid of them if you want. Before heading into the clear pipe at the end of this section, swim upward with the rising water to find an alcove with Green Star #1. Through the pipe, you’ll reach the Checkpoint Flag and another body of water. Swim down to find a power-up in the alcove to the left, then continue right. At the sign that points down, swim up instead to find a hidden area.
Here, grab the Stamp and Cannon Box before returning to the main area and continuing (you will need the Cannon Box shortly). Avoid the Porcupuffers and shoot the wall behind the enemies, where it’s visibly cracked, to reveal a pipe. This pipe takes you to a wonderful room that will have a clock item, as well as Green Star #2. Back through the pipe, head right firing at the Porcupuffers so you can clear a path. Ignore the green pipe at the end and swim up where the third column of Porcupuffers was located. Take the clear pipe to a new area, where you will find Green Star #3 on the right side. Go up the left side to the pipe, which will bring you to the Goal Pole.