From the Super Mario Wiki
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After a transition showing the player character walking into a Warp Pipe, the player will be dropped into the underwater segment, which contains Cheep Cheeps that swim through the walls. After swimming through a narrow corridor with a Blooper at the end, there is a flight of stairs with a Koopa Paratroopa jumping down, followed by a row of Coins on the waterbed with a Koopa Troopa. Next, the player has to swim through coral walls while avoiding a Blooper, Cheep Cheeps, a Koopa Paratroopa, and a Koopa Troopa. After a square cove with a Blooper in it are downward moving lifts that will push the player into the chasm below if they get caught underneath. Next is a large block with Coins underneath that is followed by a large Fire Bar, a Blooper, and a Koopa Paratroopa before the Warp Pipe to the Goal Pole.