You will have another water level here, complete with enemies called Rammerheads. These shark-like creatures are more obstacles here, and you can’t really do anything about them, as they are impervious to damage from pretty much any power-up. You will automatically be dropped into the water at the beginning, so swim right until you reach a ?-Block, which contains a power-up (Fire Flower). Continue to the right and when you see the wooden crates, break the middle one to see Green Star #1 hidden behind it. At the end of this little section, take the green pipe to arrive in the next part.
The Rammerheads here will be swimming around a central piece of land, so you will have to wait for a break, then quickly swim to the next open space. Also, keep an eye on the tails of the Rammerheads, are some of them are carrying coins in bubbles. Pay attention even more to see that one is carrying Green Star #2, so wait until you can easily grab it without taking damage. Note that there is a clock pickup at the bottom here, as well as a Super Mushroom in a crate in the upper left, but they’re not necessary to advance. On the right side of this area, take the pipe to an area with a Koopa Troopa and some Madpoles, as well as the Checkpoint Flag.
You don’t need to do anything here, so just hurry to pipe at the end. Back in the water, the Rammerheads will be rotating around a central point. You will need the Fire Flower here, as the second set of rotating Rammerheads are guarding a clear pipe with two mines protecting Green Star #3. You cannot damage boost past the mines, as they won’t disappear, so you will need the Fire Flower. After collecting the star, continue swimming upwards, bypassing the Rammerheads, until you reach a clear pipe that takes you to a water jet. This area has a bunch of Bloopers, as well as some POW Blocks to help you clear things out.
Past that area will be another one with more Rammerheads, but these will be swimming vertically. Get in the first column and swim upwards, where you should spot an alcove with the Stamp, so squeeze in there and get it. From there, enter the pipe on the right and dodge the Rammerheads by the Goal Pole to reach the top of it.