While underwater, there are several key factors integral to your survival. Although the fireball is a great projectile for dispatching enemies, the snowball’s freezing effects will often help you clear specific underwater obstacles. For example, they can plug pipes that spew jets of bubbles.
By far, the best power-up for underwater levels is Mario’s Penguin suit. While wearing it, Mario can throw snowballs, glide across the top of water, and won’t drift as he swims, making him a better swimmer!
Now go down the green pipe on the right to find a secret room with two 1-Up Mushrooms, a Fire Flower, and a Star!
One of the clams at Point 7 has a 1-Up Mushroom. Wait until it begins to open up again before swooping down and grabbing it. If you take too long and it clamps down on you, you’ll take damage. If you’re regular Mario, you’ll die!