This is another underwater level, where the current constantly pushes you. Hit the nearby ? block for a Fire Flower and go down the pipe. You will immediately be caught up in the current, pushed to the right. Hit the P switch for a path of coins to follow, then stay further up to get the first star coin. Be careful to avoid the fish along the way. When you reach the end, go down the red pipe.
Down the pipe is a split path. Go down the one to the left to enter a relatively long tunnel until you see the second star coin. Hit the switch to stop the current and hurriedly swim to the coin before time runs out. Alternatively, you can skip this portion of the level by taking the paths to the right, which lead to a maze of currents. Either way, you'll reach the midpoint flag. The nearby ? block will contain a Fire Flower. After grabbing it, go up the red pipe.
You'll enter a new area with strong currents. Carefully weave around enemies, following trails of coins when possible. Along the way is a roulette block, which can grant a Gold Flower. This will allow you to enter a pipe blocked by bricks. However, this is merely a distraction. Just follow the current until you see the third star coin, which should be easy to reach. At the end, you'll be able to exit via another pipe, which takes you to the flagpole.