Steer clear of the Rammerheads as best you can, and watch out for their wave-like swimming paths.
Green Star #1
A row of wooden crates on the ground can be destroyed to reveal the first Green Star.
Green Star #2
The next section of the level has you swimming around narrow passages while Rammerheads swim through them. However, these Rammerheads are dragging bubbles with helpful items behind them, and one of them is the second Green Star.
The Rammerheads can outswim you, and in the narrow passages that can be a problem. If it looks like they’re going to catch you, make for the closest safe spot you can see, and wait for them to pass.
The pool beyond the Checkpoint Flag is infested with Madpoles. You can use the Koopa Troopa’s shell to quickly wipe them out. Additionally, the glowing spots on the floor can be ground pounded for some extra Coins and items.
Use the alcoves on the walls to escape the Rammerheads swirling around these areas whenever necessary.
Green Star #3
To safely reach the third Green Star, you have to destroy the spike balls inside the Clear Pipe there. You can use a Fire Flower, or you can grab a Super Star from nearby to destroy the Spike Balls.
Bloopers can make this section a pain. Fortunately, you can find a Fire Flower if you need it, and the Red POW Blocks in the area are useful for defeating the Bloopers.
Stamp
The Stamp can be found by swimming higher into this room filled with Rammerheads. You’ll have to swim fast to avoid the ones coming from below and reach the Stamp’s alcove before you’re hit by an enemy. The exit to the Goal Pole zone is very close!