New Super Mario Bros. 2: 3-2

Time to go swimming! Take the pipe underwater. In this level, you'll find a periodic vortex here and there. The level is also crawling with Urchins, so take them out with fireballs. Making your life difficult are some snake platforms that get in your way, and can possibly crush you. The first Star Coin is in the second vortex. Wait until the vortex goes away to get it. If you have a Fire Flower, there's a fire pipe that will make coins in bubbles, as well as a Starman, if you give it enough fireballs.  Avoid the giant Cheep-Cheep and reach the midpoint. If, by chance, you have a Gold Flower, destroy the brick blocks over the magenta pipe. It will take you to an area with lots of brick blocks so you can make lots of coins. Go down the yellow pipe with one vortex on the left and another on the right. It will lead to several snake platforms, blue Cheep-Cheep and the second Star Coin. The P-Switch gives you several Silver Coins. Take out the blue Cheep-Cheep if you can; they'll chase you through the final part of the level, where you'll be dealing with snake platforms and Urchins floating up and down. The third Star Coin is in this area. The pipe at the end takes you to the flag.

Coin Total: 15,001

New Super Mario Bros. 2: 1-5

A Gold Block appeared on the map. We're going underwater this time. Near the blue Cheep-Cheep and some bricks lining a gap below is the first Star Coin. There's a random coin block so you can try your luck. We take a yellow pipe down by accessing through a ring of Cheep-Cheep from the right side. The second Star Coin is down in a strong current, so swim up quickly once you get it. Take the green pipe up and you'll be right next to the midpoint. There's a random item block. Swim up through a tiny gap in the ceiling past the rings of Cheep-Cheep for the third Star Coin. There's a giant Cheep-Cheep near a P-Switch that makes Silver Coins. Avoid it. Nearby is the pipe to the flagpole.

Coin Total: 2779

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.