You will find this secret star unecpetally in the Buoy base and this secret star is acually a green star! I'm going to walk you through it. When you first arrive go to the left side of the water and dive in you will see this cage with something green in it, it will acually be a warp pipe get one of those missles to follow you to break the cage, you'll propally think how to get in there will be a small whirlpool that will lead you into the warp pipe. The warp pipe will take you outside of the ball of water ( but if you fall off it'll take you back to where you were before you entered the warp pipe so be careful ). There will be cannons with bullet bills in them, but you can only get one to follow you or they will collide and crash, get to the end you'll see a cage with the green star in it! Get the bullet bill to crash the cage and VOLLA! there's a green star!