Right off the bat, dive underwater and swim through the tunnel to pop out at the submarine bay section of the course. Once all three chests are opened, the power star will appear underwater for you to grab. If you get sucked into the whirlpool you'll get spit out of the level and be forced to retry. Once you've got that one opened, carefully open the treasure chest that's closest to the whirlpool.
The second chest to open is the one that's covered with seaweeds. Open it up, grab the air bubble that rises out of it, and aim for the next. Swim down to the bottom and look for the first chest-it's the one that's furthest away from the other two, and is sitting next to a clam. There are three treasure chests sitting at the bottom of the pool of water, and you've got to open them up in the correct order to reveal the golden power star.
