Those extremely familiar with Jolly Roger Bay will get a sense of dejavu with this star, as you will need to open four chests in the correct order. Opening the wrong chest will not only electrocute Mario, but the puzzle will be reset. Unlike the previous stars centering around chests, you also have to worry about the current from the whirlpool for this one. When you begin the stage, swim down and look for a Clam next to two columns of coins. There will be a chest to the left of that Clam, which will be the first one you open.
Swim on over towards the tunnel and locate the chest that has some ungrouped coins near it, which will be the second chest you open. With those two out of the way, there’s only two more remaining. The next one you want to open is separate from all of the others; if you’re facing the Clam from before, it’s to the right of it. So, that will only leave one chest, which is near the tunnel, but right next to the whirlpool. You will want to be extremely careful here, since it’s close to the whirlpool. Once you open the fourth chest, though, the star will spawn to the left of the tunnel.