New Super Mario Bros. U: Sparkling Waters-5 -- Dragoneel's Undersea Grotto

Take the middle pipe down to an underwater level with quicksand. There are Cheep-Cheep in the level, as well as Dragoneel, a long eel that resemble Gobblegut that follows us around. You can use fireballs to slow it down or speed it up, depending on if it's in front of or behind you. The first Star Coin is over the horizontal pipe with the current. Approach it from the upper right to get it. Avoid Dragoneel and keep swimming and you'll find the midpoint. The second Star Coin is surroundeed by brick blocks, so you'll have to be at least Super to get it. Grab some Red Coins if you want and go down a red pipe. Drop down for the third Star Coin and quickly return via the same pipe before two Dragoneels appear and make life difficult. Before you reach the pipe to the flagpole, there's a swarm of Cheep-Cheep. This opens the path to the castle and a Mushroom House.