Turtle Zone Area 1
From the Super Mario Wiki
Mario starts the level on a raised ledge by some water. Inside the water are Blurps and Cheep Cheeps, and shortly before the first Blurp is a raised ledge with a ? Block containing a power-up. After this is a piece of land that Mario must swim under, avoiding fish, a Shark, and a Spiny Cheep-Cheep. Near the checkpoint bell, there are more ? Blocks, Sharks, and Blurps. Next, Mario must zig-zag through multiple chunks of ground, with the second to last one having a ? Block holding a power up. Mario must then avoid Cheep Cheeps and Sharks while swimming through columns with narrow gaps. After this is a branching path: the bottom path has two ? Blocks that only Small Mario can fit under, with one holding a coin and the other holding a 1-Up Heart, and the bell that ends the level is located on the top path.