New Super Mario Bros.: 6-5

A pipe takes us to an underwater level. The green Cheep-Cheep will chase you, so swim quickly or take them out with fireballs. The first Star Coin is near the ceiling. Shortly after is a Red Coin Ring. You'll find a vortex or two in this level. They'll be periodic, so they'll disappear after a short while. Use this time to swim past them and up a yellow pipe between two of them. The 2nd Star Coin is here. Once you return to the main level, you'll reach the midpoint. Nearby is an item block shrouded in seaweed. It has a Starman to make your life easier. Towards the end of the level is a vortex with brick blocks at its base. Go down it to reach the 3rd Star Coin. The pipe will leave you near the end pipe and a giant green Cheep-Cheep. Take the pipe to the flag immediately.