New Super Mario Bros./World 4-3

You've stumbled upon the one body of water that isn't tainted by lethal poison. Alas, this lengthy river is the domain of the Unagi, eel creatures that can grow up to humongous sizes. They will gladly take a bite out of Mario, and the bigger ones can't be defeated at all. Stay out of the way of these evil eels!

Level walkthrough
Descend down the hill and hop into the warp pipe to begin the stage. The level starts quite peacefully, with several Cheep Cheeps mindlessly swimming by. However, the screen will soon start to scroll by itself. You might have experienced forced scrolling back in World 1-A, and this course poses a similar danger of getting crushed between a wall and the screen. There is an even bigger danger here, though. A Mega Unagi will charge to the left, covering the entire top half of the screen. It is completely invulnerable, so just duck under it until it passes.

After the coast is clear, swim up and over the rock formation. You can see a Star Coin in a small crevice. Quickly swim toward it and collect it to avoid getting crushed from the forced scrolling. Up ahead, there is a red ring, as well as an incoming Mega Unagi coming through the bottom half of the screen. The red coins are mostly located out of the path of the Mega Unagi, but some bring you close to some regular Unagi. These smaller eels are stay within the walls, but they will snap at you when you get close. You'll soon pass the checkpoint flag.

The two blocks after the flag contain some nice items. The first has a power-up, while the second one contains a 1-Up. Another Mega Unagi will cross over you. Once it passes, swim past the wall and sink down into the small underwater valley where the second Star Coin can be seen. The two Unagi nearby are a very real threat, so be precise with your positioning. You will soon see a P-Switch, as well as some pipes that blast bubbles out. A Mega Unagi will soon swim across the top, which the pipes most certainly push you into. The P-Switch temporarily disables these pipes, an effect necessary to stay out of harm's way.

Among this series of pipes, the last one is actually a warp pipe. It will take you to a secret area with the third Star Coin. The bubble-blowing pipe next to it poses a problem, which can be circumvented by going around the room in a counter-clockwise manner, letting the pipe propel you into the coin. The Unagi are still a worry here, both when getting the coin and swimming back to the warp pipe. Back in the main course, you will have to contend with one last Mega Unagi, this one crossing the bottom of the screen. After that is a vertical shaft flanked by Unagi on both sides. Stay within the center and swim up, then keep going until you see the exit pipe. Go out of it and hop up the steps, stomp the Goomba, and jump to the flagpole to finish.

Star Coins

 * 1) This coin is in a crevice, shortly after the first Mega Unagi. Get it quickly to avoid getting crushed by the auto-scrolling screen.
 * 2) The coin is found soon after the checkpoint flag, positioned between two Unagis. Carefully sink between them to obtain it safely.
 * 3) After hitting a P-Switch, go through the rightmost warp pipe nearby to enter a secret area. Go anti-clockwise around the center block to grab the coin.