It’s much easier to swim through the area when you’re able to clear enemies out of your way. Make sure you grab at least one of the available Fire Flowers.
The first Star Coin is located at Point 2. The Spiny Cheep Cheep can be fairly aggressive, so use your fireballs to deal with this enemy before you sink down to the Star Coin.
Swim over the platform at Point 3, then circle back and enter the yellow pipe at the bottom of the screen.
After you pass through the pipe, sink down toward the Star Coin. Swim against the current to avoid being pulled off of the screen, but allow yourself to drop low enough to collect the Star Coin. As you fight your way back out of the current, watch out for the Cheep Cheeps swimming through the area.
The last Star Coin is located in a hidden area near Point 4. When you reach this spot, swim up through the narrow opening along the top of the passage.
Swim up to the water’s surface, then leap up to grab the Star Coin.