New Super Luigi U: Rock-Candy Mines-2 -- Porcupuffer Cavern

Take the yellow pipe past the crab. The main part of the level bobs in the water. There are music note blocks that allow you to reach a pole. Bounce on the third music note block from the left to trigger the appearance of the first Star Coin overhead. Keep going and you'll see another pole near a crab. Jump up to the right to find a hidden area with the second Star Coin and 8-bit Luigi. The third Star Coin will appear below. Take the pipe at the end and bounce on the music note blocks to reach the flagpole.