When you pass through the first pipe, a strong current sweeps you to the right. As it does, sink down low enough to bump into the platform at Point 1. Slide down and hit the P Switch below you to reveal a trail of blue coins.
The first Star Coin is located above the platform at Point 2. The current is very strong, so make sure you’re in position to grab it on your way through the area.
When you reach Point 3, swim through the hole in the floor. This route is a bit longer, but there are some useful items within the hidden area.
Hit the ? Switch at Point 4, then swim down to collect the Star Coin. If the currents reappear before you reach it, swim back up and reactivate the ? Switch.
If you have a Gold Flower, use it to clear the Blocks at Point 5. Enter the exposed pipe to find a hidden room filled with Blocks.
As you approach Point 6, sink toward the bottom of the screen and allow the current to carry you through the Red Ring. After it does, follow the trail of coins as it leads to the top of the screen and zigzags through the area. Doing this should keep you in position to collect all of the red coins.
The third Star Coin is floating at Point 7. Avoid the Small Urchins, but make sure you’re high enough to collect the Star Coin as the current pushes you through the area.