The first Star Coin is located in a hidden area. To find it, enter the yellow pipe at Point 1.
After you pass through the pipe, jump across the water in the center of the area. When you’re directly above the Star Coin, perform a ground pound. This move allows you to overcome the water currents that would otherwise prevent you from reaching the Star Coin.
Pass through the Red Ring at Point 2 to reveal a trail of red coins to the right. Follow the coins down to the cavern floor and up through the passage to the right. The last red coin floats away as you approach it, so keep swimming until you catch it!
If you brought a Gold Flower (or received one from the Roulette Block near the start of the level), use a gold fireball to destroy the Blocks at Point 3. Enter the exposed pipe to find a room filled with Blocks.
The second Star Coin is floating at Point 4, but a row of Invisible Blocks prevents you from reaching it. Continue to the right, then swim up to the cavern ceiling.
When you reach the row of ? Blocks, swim down through the opening to the left and approach the Star Coin from above.
The third Star Coin is in a hidden area. To find it, enter the pipe at Point 5.
After you pass through the pipe, swim up and hit the ? Switch to deactivate the water currents flowing from the pipes across the room. Dodge the Cheep Cheeps swimming through the area, and grab the Star Coin before the ? Switch wears off.