Duck as the platform pulls back to the left, then jump up to get the Star Coin. Wait for the spiked beetles to roam by, then jump onto the shifting platform to the left. Star Coin 2: Well past the halfway point of the stage, look for the Star Coin on the far left side of the tower (just beyond the low ceiling of spikes). Wait for the nearby platform to slide to the left so you can use it to boost Mario to the prize.
Star Coin 1: The first Star Coin is easily had by exploring the right side of the screen very early in the stage. Get rid of 'em and you won't have to worry about the problem. If you leave the skeeters be, they'll drop bombs into the water as you try to chase it upwards in the rooms. Level Tip: Aside from the area with the second Star Coin, it's best to get rid of the skeeters before you start messing with the water levels. Before the water recedes, jump to grab the Star Coin. Hit the switch in the center of the room and then follow the passage on the right. Star Coin 3: Late in the stage you'll enter another underwater portion with two skeeters overhead and two smiling yellow blocks. Alternatively, Mini Mario can wall jump to grab the Star Coin without the help of the skeeters.
You need to jump off the back of a skeeter to reach the Star Coin overhead. Leave the skeeters alone as you hit the switch and follow the water as it rises. There are two skeeters overhead and a nearby switch. Star Coin 2: After crushing through the stacks of brick blocks, go through the pipe to the right to enter the fourth area. Once you drop past the Star Coin there's no going back. Alternatively, you can slowly whittle through the brick blocks to more cautiously aim your descent. Ground pound through the blocks and you should smash straight through the first Star Coin. Stand over the fourth column of brick blocks, counting from the right. Make your way to the third and final brick block stack and prepare yourself for a ground pound the likes of which has never before been seen. There are three large stacks of brick blocks.
Star Coin 1: In the third area of the stage, you'll drop through an area that's almost completely filled with brick blocks. Wait for the large cannon to fire before wall jumping off of the large cannon and back onto the upper walkways. To get it, simply jump down between flying bullets. Star Coin 3: The third Star Coin appears just after the second. Star Coin 2: Jump off a flying bullet bill to gain the height required to grab the second Star Coin. Drop the springboard on the platform and use the springboard to jump and get the first Star Coin. Grab the springboard and carry it to the short two-block platform to the right. Hit the question block from below (stand on one solid block underneath, then jump sideways to land on the second) to knock out the springboard inside. Star Coin 1: A little under halfway through the stage you'll see a question block floating over the top of a gap between two solid blocks.