World 8-4 (New Super Mario Bros. Wii)
From the Super Mario Wiki
After falling down a deep tunnel, Mario and company have to deal with several Stone Spikes. There are Bulbers in the water, which help the player see. The second part of the level is infested with Jellybeams. They are the only light source in this section of the level. For the final part, the player has to swim upwards through some pipes that make strong water currents. After entering a pipe at the end of the area, the flagpole can be found.
There is a secret area in the level, which can be found underneath the last Stone Spikes in the second room. It has several coins and a Spiny Cheep Cheep.