If you don't know the spoiler boxes very well, just click on "Show Spoiler" below me where it says "Spoiler alert!" These help prevent someone seeing a spoiler if they wanted to figure it out themselves.
Spoiler Alert!You have to go through the first gate of the level, then go underwater ALL the way to the bottom where you see some green koopa shells. Grab a shell, then go over to where a broken ship and some mines are. Hit one of the mines and they all explode. A Launch Star will appear. Ride it to arrive in a big purple box. You have to smash the crystals on the wall to make light appear and get the Boo to fall in the light and you get your star.