New Super Mario Bros. U: Soda Jungle-5 -- Deepsea Ruins

This level has water that's safe to swim in. There's an Ice Flower as well as a Fishbone outside, amongst the Stone Eyes. Platform on the Stone Eyes and dodge the Boo ring, then drop in the water between the Stone Eyes facing each other for the first Star Coin. At the end is a pipe that leads to a dark underwater area. You can't see much, so proceed cautiously and leave the light-up jellyfish alone. There are some lights in the water, one of which is in a Boo Ring. This is a vertical level, so you'll be swimming to the top of the level. After the ring of Boos is a Red Coin Ring. At the top of this part is a pipe to the next area, which has the midpoint. There are glowfish in this area, so leave them alone. You'll need all the light you can get. A little higher in the level, on the right side, is an item block with a Starman. This will light up the entire level, so use its power wisely and make some serious progress. The second Star Coin is in the center of a ring of Boos. As you get closer to the top, the Stone Eyes on the walls will start to move. Wait for the top Stone Eye to move into place so you can reach the pipe at the top. This will take you to the regular goal. This opens a path that double back to the path between here and the Ghost House, like the path for Soda Jungle-4. If you return to the level, at the top near the pipe to the regular goal, go over that last Stone Eye and to the right for the third Star Coin, as well as a pipe to the red goal. This opens the path out of this part of the jungle, through the courtyard of Boos, to Soda Jungle-6 and -7. (To get through the courtyard, memorize which traps doors the Boos go into, then avoid them on the path out of there.)