This stage is Pipeline Lagoon (3-5). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars. We will also note a secret 1 UP.
Game Play Walkthrough Video
Green Star 1
You will come across your first green star at the big bulb looking pipe with the arrow. Swim to the top to reveal a hidden green star.
Green Star 2
You will need the cannon box at the hidden room with the stamp. Dive down to the hole and blast the crack in the wall. Get into the pipe to another room with the second green star.
Green Star 3
The last star is near the end of the stage where you will see a few groups of spike fish. It is at another hidden hole with last group, instead of going down the green pipe, go through the hidden pipe. The last green star can then be found at the upper right.
After reaching the Checkpoint Flag, head right. When you reach a place where you drop down the stage, instead of dropping swim up for a hidden room. Grab the stamp and keep the cannon.