World 8-2 (New Super Mario Bros.)
From the Super Mario Wiki
This level is an underwater level, somewhat unusual for a final world. The water within the level can be made to rise upwards using ? Switches, and lowers on its own after a set amount of time passes.
The level starts with the player outside, and a Warp Pipe which takes them into the main level. In the first section of the level, the player must use a ? Switch to make the water rise up, while avoiding a Skeeter and its projectiles. At the top of the area is a pipe leading to the next section, along with a ? Block containing a Fire Flower. The next section contains barrels floating atop the water, along with more Skeeters. This section also contains a Mini Warp Pipe, which allows the player to skip a portion of the level and go into a bonus area; however, using this shortcut renders the player unable to retrieve the first Star Coin. After using another ? Switch to make the water rise and another pipe accessible, the player is taken to the next section. This section is full of Brick Blocks, which can be Ground Pounded in certain locations to earn coins. Once the player reaches the bottom of the area, the level's midway point is reached.
The next area of the level contains two Skeeters and another ? Switch; the player must press the switch and avoid the Skeeters to reach another pipe that takes them to the next part of the level. This area contains two more Skeeters, blocks, and ? Switch below a wall which divides the area into two sections. Pressing the ? Switch and heading left takes the player to the last pipe, which leads outside again; the player must jump on top of wooden blocks to reach the flagpole.