New Super Luigi U: Sparkling Waters-2 -- Urchin Reef Romp

The main part of the level is underwater, but before you take the pipe underwater, run past the pipe, wall jump up, then run to the left for the first Star Coin. Go through the pipe to go underwater. You'll find giant and small Urchins, as well as Cheep-Cheeps. There are lots of currents in this level. The second Star Coin is in a current, under a giant Urchin. The third Star Coin is guarded by Urchins, so use a Fire Flower to kill them, then get it. Hit the P-Switch for Silver Coins through the series of giant Urchins to the pipe that leads to the flag.