World 8-3 (New Super Mario Bros.)
From the Super Mario Wiki, the Mario encyclopedia
The player starts outside the main level, with a pipe taking them inside. In the very first section are two Bumpers surrounding a ? Block containing a Fire Flower. As the player swims upwards, they encounter various Unagi; some stay in place and lash out when the player gets near, and some travel horizontally. After passing through a group of Unagi and a Red Ring, the level's midway point is reached rather early on.
After the midway point, a Mega Unagi is seen on the left side of the screen, and as soon as the player becomes level with it, it will chase after them. The Mega Unagi will chase the player through the rest of the level, and will destroy all Brick Blocks in its path; the player must swim quickly to avoid it, while also dodging regular Unagi, Cheep-Cheeps, and Bumpers along the way. The player can use pipes with rushing water to boost their speed and get farther away from the Mega Unagi.
If the player stops swimming, the Mega Unagi will stall with them. After a certain amount of time passes, the Mega Unagi will try to swim into the player, and then swim away offscreen. The Mega Unagi will not respawn as long as the player doesn't stall again, meaning the rest of the level will become much easier if the player survives.
At the end of the Mega Unagi chase section are two pipes; a green pipe on the lower end, and a red pipe above it. Both of these pipes will take the player to the flagpole, though the red pipe will allow the player to reach the top of it by stomping on a single Koopa Paratroopa.