This is the first underwater level in the game, but you start on the beach and bringing a Fire Flower along isn’t a bad idea.
Cross the water past the Goombas and jump on the invisible Coin Coffer in the bushes to the right of the pipe for a few quick coins. Take the pipe to reach the next area where you can climb up the wall with the Cat Suit to a tree that has a Super Mushroom . Into the water you’ll find a Roulette Block with the chance to grab a Fire Flower or Super Leaf preferably. Beware the Goombas and Cheep Cheeps as you move through the water, they aren’t difficult to deal with but it’s easy to get complacent. The enemy you really need to be careful of is the Porcupuffer, best defeated with a well-placed Fireball.
Before you head into the pipe at the end of the waterway, swim upward with the rising water to Green Star #1 . Through the pipe you’ll reach the Checkpoint Flag and another body of water. Swim down to find a Fire Flower in the alcove to the left then continue right. At the sign that points down, instead swim up! Grab the Stamp and Cannon Box before following the signs guidance.
Avoid the Porcupuffers and shoot the wall at left where it’s visibly cracked to reveal a pipe. This pipe takes you to a wonderful room filled with coins , Gold Rings , a Green +Clock , and Green Star #2 .
Back through the pipe, head right firing at the Porcupuffers so you can clear a path to swim up, disregarding the green pipe. Take the clear pipe to Green Star #3 and up to a Green +Clock then into the pipe that leads to the Goal Pole.
After clearing this level investigate the ground on the far side of World 3-6 and dig up a pipe that leads to a 1-Up Mushroom .