The first Star Coin is hidden in a secret room near the start of the level. Drop into the water to draw the Fishbones toward the right wall, then hop over the enemy and swim to the small hole on the left edge of the floor.
When you reach the next room, dodge the Fishbones and stone slabs to collect the Star Coin near the left wall.
The second Star Coin is located at Point 2. Wait for the Fishbones to swim to the left, then sneak up to the Star Coin. The enemies will attack when they turn around—draw them into one of the nearby slabs.
To use the secret exit, you must bring a Mini Mushroom into the level. Reach Point 3 as Mini Mario, then enter the small pipe to the right. Navigate the stone slabs and Fishbones along the alternate route to reach the secret Goal Pole.
When you pass through the Red Ring at Point 4, red coins appear in the surrounding water currents. Move between the water currents to circle the room and collect all eight coins before they vanish.
The third Star Coin is located in a hidden room. After you deal with the Red Ring, enter the green pipe to the left.
After you pass through the pipe, drop out of the water current and collect the Star Coin to the right.
The final chamber contains four Reznors on a single wheel. There are no vacant ? Blocks when you enter the room, and after you defeat at two Reznors the floor starts to crumble. Clear out all four enemies to complete the level.