The first Star Coin is located at Point 1. Swim down and grab it before the nearby platforms emerge from the ground and block the path.
Be particularly careful as you swim through Point 2. The platforms below you can knock the nearby Urchins right toward you.
The second Star Coin is floating behind the Urchin at Point 3. Wait for the enemy to drop out of the way, then swim up and collect the Star Coin.
The third Star Coin is located in a hidden area. To find it, enter the green pipe at Point 4.
After you pass through the pipe, swim toward the ceiling. The area is packed with enemies. If you still have a Fire Flower, try to pick off a few of your pursuers, but make sure you keep moving. Collect the Star Coin before it’s covered by the coral slab sliding across the ceiling.
If you have a Gold Flower, you can use it to destroy the Blocks at Point 5. Enter the exposed pipe to find a room filled with Blocks.