New Super Mario Bros./World 1-A

Going down the southern path leads you to a lake that you must swim across. The screen moves forward as you do, and can crush you against the reef's walls. You're defenseless without the ability to stomp, so a Fire Flower is certainly valuable here. Pairs of Sushi will harass you throughout the stage, stay between them to remain safe!

Level walkthrough
You begin on land, so go down the warp pipe to dive into the lake, where the majority of the level takes place. Once in the water, you must now watch out for the Cheep Cheeps that swim around aimlessly. The only way to defeat them underwater is with fireballs, so try to preserve your Fire Flower. The middle of the first set of blocks contains a power-up, which can give you a Fire Flower if you're already Super Mario. Once you start progressing forward, the screen will automatically scroll to the right. Now you must stay away from the level's walls, as you can get crushed by it if the screen scrolls to far forward. In addition to the danger of crushing, there are also pairs of sharks called Sushi that will attack you regularly. They appear on the right side of the screen and swim in a straight line to the left. You can easily dodge them by staying between two Sushi, but they can be a bit of a nuisance when collecting coins. Once again, a fireball can take a Sushi out instantly, just like any other aquatic creature.

Other than the stationary coins suspended in the water, you can also collect coins drifting by in bubbles. Soon, you will get to a P-Switch. Bump into it to cause a ton of silver coins to appear in the following section. The coins are located around a bunch of huge bubbles, which can bounce you away and make it hard to move around. After the bubbles is what appears to be a mostly empty area. In reality, there's actually some hidden blocks scattered around here, one of which contains a power-up and two that gives you 1-Ups. Additionally, there is also a Star Coin that is very visible, just waiting for you to swim up to collect it. After the first Star Coin is a warp pipe. Go into it to get transported to an above ground area where you can get the second Star Coin. Simply jump on to the blocks from the pipe to get to it. Go in the right warp pipe to return underwater.

A roulette block marks the midway checkpoint of this stage. These blocks are a bit peculiar, as they can grant you any item except a Mega Mushroom, including a Star or a Blue Shell. It is difficult to get the exact item you want though, since each option scrolls by very quickly. A short while after the block and a red ring will scroll on to the screen. Wait for as long as possible before going through it so that you can get more time to collect each of the coins. The position of each red coin is pretty simple and straightforward, but some can put you in the path of a Sushi. Past the red coins are a series of pipes that have water currents that push you in the direction the pipe is facing, which can thrust you into a Sushi. A second P-Switch will soon appear, and hitting will make a huge amount of coins cover the area ahead, more than enough for you to get 1-Ups. After the silver coin section is the final Star Coin, wedged between two walls. You can collect it pretty simply with small Mario by swimming through the gap, while it is trickier to obtain if you're big. Note that you are in constant danger of getting hit by a Sushi while getting the Star Coin. A bit further to the right is the warp pipe that will take you back above ground, where the flagpole is.

Completing this level simultaneously lets you get to the World 1 Castle as well as a red Toad House. While you can attempt to beat the castle right away, it doesn't hurt to try out World 1-5 if you have yet to do so!

Star Coins

 * 1) Out in the open a bit after the section with large bubbles.
 * 2) Easily obtained in an above water sub-area, reached via warp pipe.
 * 3) Between two horizontal walls near the end. Can be hard to get if you're not small Mario.