The first underwater level in the game.
Before the Checkpoint
The level begins in a small pool area. Invisible coin-purse enemies are hidden in bushes in the lower-left and upper-right for an easy 20 coins. When you're ready, head through the pipe.
Head right and dive underwater to find a Roulette Block for a random power-up. Head farther right and continue along, dodging Cheep-Cheeps and Porcupuffers. Before taking the exit pipe, swim up to find the first green star. Then take the pipe past a pair of sunken ships and to the checkpoint.
After the Checkpoint
Dive underwater and hit the first ? block for a fire-flower. Swim right and up from there to find a hidden area containing the level stamp and a cannon-block (and an invisible block with a mushroom). Grab the cannon block and head straight down to find a cracked wall. Shoot it and enter the pipe for a treasure room with the second green star.
After exiting the treasure room, head right to find a bunch of Porcupuffers, and swim directly up from the third column of them to find a hidden pipe. Taking this pipe will lead to an area with the last green star and 2 more coin-purses (in the clumps of grass). Furthermore, this route will put you on the upper ledge of the goal area, allowing you to reach the top of the flagpole.