Land and jump over the ?-Switch to the Glow Block. Pick it up and then hit the Switch. Follow the Coins as you drop down to the water below, then throw the Glow Block and swim to the pipe.
On the other side are some Bulbers and Spikes above. The angler fish will do damage if you touch them, but they won't go out of their way just to hurt you. Swim through them and hit the ? Blocks for a Fire Flower, then swim past the Spikes and avoid the rocks they throw. After four or so Spikes there is a gap in the ground they're standing on. Swim up and use fireballs to take them all out. On the far left of this ledge is Star Coin #1. Run back right across the ledge and wipe out the ones you missed before dropping back into the water. Grab the Coins around the block and enter the pipe.
There are some Jellybeams above you that cast spotlights down through the water to show you what is underneath. When you get close, however, they slowly drop down, then move back up, much like Thwomps. Hit the Blocks between the first and second for a Fire Flower. Swim right further to come across some Bloopers contained within blocks. Swim up past some Jellybeams to find a ?-Switch. Hit the Switch and swim frantically past the Bloopers to Star Coin #2 below. Use fireballs if you can to wipe them out, or swim right a bit further to a ? Block and use the Star that comes out to grab the Star Coin quickly and escape right. There is another ? Block nearby which has another Star IF you can hit it while the first Star is still in effect. Otherwise, you will get a Coin. Slowly float down below the pipe to a Brick Block under a red ledge. Hit the Block for a 1-Up Mushroom and then swim up, making the Jellybeam above drop down. Enter the pipe when it is out of your way.
Wait for the Jellybeams above to drop, then swim back up with them to two ? Blocks, one of which has a Fire Flower. Swim left and up, but watch for the Jellybeam here as well. There is a green pipe expelling water to the right, and above it are two more Jellybeams. Get past these and the next pipe to another pipe and some more Jellybeams, then some more pipes. At the surface of the water is another green pipe, except this one will take you out of this cave. Don't enter it just yet. Under the pipe to your left is Star Coin #3. Grab it and then exit.
Jump up the ledges to your left, then run from the wall and jump to the Brick Blocks, double to the next Blocks and triple on to the Finish Flag.
There is a secret area in the level, which can be accessed through a Warp Pipe underneath the last group of Stone Spikes in the second room. It contains several coins and a Spiny Cheep Cheep.
- Star Coin 1: The first Star Coin is located in the second area, atop the long platform with the numerous Stone Spikes. Mario and co. must swim to the surface, jump on the platform and avoid all of the Stone Spikes to get the Star Coin.
- Star Coin 2: In the third area, the second Star Coin, along with four Bloopers, are caged individually by several blocks. Mario and co. must activate the nearby ? Switch to make all of the blocks disappear and make the Star Coin accessible.
- Star Coin 3: In the fourth area, the third Star Coin is in a small opening in the rocks, to the left.