Take a dive into a mountain lake and enjoy the cold, fresh water. This lake is home to the Deep Cheeps, a subspecies of Cheep Cheeps that are especially aggressive. Whirlpools will rise up as you weave and dodge between the schools of fish. Don't get sucked in, or you'll be dragged to your watery grave.
Dive into the pipe to start the level. Start swimming to the right, hitting the trio of blocks if you need a power-up. A Fire Flower is as useful here as in any other water stage, giving you a means of self-defense against the level's enemies. After passing by some docile Cheep Cheeps, you will encounter Deep Cheeps, green-colored Cheep Cheeps that actively home in on your position. They will give up chase once you swim behind them, so swim forward as quickly as you can to avoid them.
After swimming over a bottomless pit for a while and evading a few more Deep Cheeps, you will see the first Star Coin right within reach. However, there are two Deep Cheeps swimming near the Star Coin. Get behind them to make them lose interest before attempting to collect the Star Coin. Soon after, there will be a red ring you can pass through. The red coins it spawns are grouped in pairs, an alternate between being on the top of the screen and the bottom. Deep Cheeps will come at you from behind around this point. Stay away from them until they pass by you.
Past the red coins, you will swim over a whirlpool. Normally, they appear as tiny rotating water currents over a gap, but regularly grow big enough to cover the entire width of the course. If you happen to be within the currents, start tapping the Jump buttons to avoid getting sucked to the bottom, which results in instant death. The whirlpools are also dangerous when Deep Cheeps are nearby, as they severely restrict your movement while you're dragged by the currents, making you an easy target for the Deep Cheeps.
After the whirlpool is the checkpoint, followed by a stretch of water with a bottomless abyss beneath it. Once you can see ground below you again, look out for a lone question block. Hitting it makes a Star come out, which lets you hit any of the Cheep Cheeps and Deep Cheeps with impunity. The three question blocks following this one will also contain Stars if you hit them while invincible. This will let you remain invulnerable for quite a while, potentially racking up a bunch of 1-Ups by defeating Deep Cheeps.
Once you pass another whirlpool, you can see a yellow pipe entrance in the ceiling. Go into it to reach a cave with a bunch of coins, and most importantly the second Star Coin at its center. Deep Cheeps will regularly swim by in both directions, so you have to be really careful to avoid touching any of them. Go out the way you came and continue onward. You must swim over two more whirlpools which have coins hovering in their path. Collect the coins if you want, but you'll have to watch out for both the whirlpool and nearby Deep Cheeps.
After a minor bottleneck, you will pass over the last whirlpool of the stage. This whirlpool has brick blocks forming the floor around it, almost as if it is artificial. In fact, if you dive into the gap between the brick blocks, you will end up in a secret area with the third Star Coin. Go through the red pipe to return slightly ahead in the level. Two Big Deep Cheeps make a brief appearance here. They behave the same as normal Deep Cheeps, and shouldn't be much of a threat this close to the exit. Go into the pipe and use the falling platforms to jump to the flagpole.
- The Star Coin is out in the open, near a Red Ring. Just swim to it to obtain it.
- When you come to a yellow pipe on the ceiling, enter it. You will be transported to a cave with the Star Coin. Watch out for the Deep Cheeps.
- The last whirlpool in the level has brick blocks at its edges. Let yourself get pulled in to end up in a secret area with the Star Coin.