This is an underwater level. Start it by jumping down the red pipe. When you go underwater, the first thing you will see is a group of ? blocks above a whirlpool that appears and disappears. The second block from the right contains a fire flower. use it to dispatch of enemies. Soon, you'll see a green pipe below. Inside, there is a roulette block, which can grant you one of several items. When you emerge through the pipe to the right, go a bit left to find the first star coin in a whirlpool. Carefully swim in and out to get it.
Meanwhile, there is a pipe to the right that spews coins when fireballs are shot into it. Send in enough fireballs, and eventually a starman will come out, granting temporary invincibility. This will be useful swimming through the next area, which contains enemies and Snake Blocks o impede your path. You should then reach the midpoint flag. Hitting the first ? block you see will grant a Fire Flower. Ahead, you'll find a yellow pipe guarded by snake blocks and whirlpools. It may be risky, but swimming in here will reveal a maze of snake blocks and a single enemy guarding the second star coin. With a power-up, the enemy can easily be dispatched of and the coin grabbed.
Back on the main level, you'll find a P switch that creates a path of coins. Swimming along this path will expose you to several enemies that will chase you, so be brisk! Eventually, you will enter an area with snake blocks and urchins. Carefully navigate through this area, and avoid being hit by either the urchins or any pursuers you may still have. You'll see the third star coin above you on the way, so grab it if you can. Once you're done with this area, you can go throguh a blue pipe to reach the surface again. Jumping on the far left of the platform to the left will reveal a hidden block containing a 1-Up. Then it's a simple matter of jumping onto the flagpole to the right.