New Super Mario Bros. U: Rock-Candy Mines-2 -- Porcupuffer Falls

To access this level, hit the red switch for the blocks that make a path to it. Take the pipe to part of the level which is underwater. The Porcupuffer will appear soon to make your life miserable. If you've got a Fire Flower, you can get rid of it for a few seconds. Swim up and get out of the water. There's a sign with an arrow pointing down. There's an invisble block on the right wall nearby. Use it to go up and over, and drop to the first Star Coin. Make sure the Porcupuffer's not directly below you. Go past the Cheep-Cheep and the pipes with currents. There's a Red Coin Ring before the midpoint. Between two red pipes are Urchins and the second Star Coin. Get it carefully. Above the random item block is an invisible block with a vine. It leads to some coins and allows you to drop down to the third Star Coin. The pipe nearby, at the end, will take you to the flagpole. This opens the path to the Fort and a set of two switches.