- "Under the Sea: In World 1-A, Mario finds himself in an underwater world that scrolls automatically. Luckily, Koopa Mario is an excellent swimmer!"
- ― New Super Mario Bros. official website
This level is auto-scrolling, so Mario will have to be careful. At the beginning of the level, Mario will find himself in a grassy area with some water and a Warp Pipe. If Mario enters the pipe, then he'll go into the water, with some Cheep-Cheeps and Blocks. After this, Mario will find several coins in air bubbles floating around the level, and some sharks known as Sushis will attack Mario. A P-Switch is just up ahead, and if Mario presses it, many Blue Coins will appear. In this area, there are several bouncy obstacles. Past this, there is the first Star Coin in the level, but it isn't too easy to get since there are a bunch of Hidden Question Blocks blocking the coin. Just after this, there is a pipe which Mario can enter to get above the surface, where he will find the second Star Coin. There are also several Question Blocks and Brick Blocks here. After that, Mario will find more Cheep Cheeps and Sushis, and a Roulette Block. Then Mario will come across a Red Ring, and he will have to be careful while grabbing Red Coins, as Sushis will attack him. Then, there are several pipes that propel Mario up. Then there is then another P-Switch which will make more Blue Coins appear. Then the third Star Coin can be obtained, but Mario must be careful, as he could be crushed between the walls of the auto-scrolling level and the actual walls. After all that, Mario will exit the underwater area with a green pipe, and he will slide down the flagpole.
- Star Coin 1: In plain sight, however, don't try to collect it from underneath because there are hidden Question Blocks.
- Star Coin 2: After the first Star Coin, go up the Warp Pipe and you end up above the surface. The Star Coin will be on some Brick Blocks and you have to use the other Brick Blocks to reach it.
- Star Coin 3: This Star Coin will be boxed off and have two entrances. The entrance on the left is a small gap so you will need to be small Mario. While the entrance on the right is a larger gap for Mario, you have to swim under the box and avoid the incoming Sushi. Also, taking the right entrance is riskier due to auto-scroll.