Enter a flooded cave to reach the Unagi's main habitat. Stay away from reef walls or you'll get snapped by a peeking eel. A humongous Mega Unagi lurks inside this cave, and it is hunting Mario down. Be sure to act fast and swim faster to avoid falling to this massive predator.
Go into the pipe at the start to be transported underground, and underwater. Once you emerge into the cave's pool, hit the question mark block between the two bubbles for a power-up, as you'll need that extra survivability. Now, you must begin your treacherous ascent up a vertical channel teeming with Unagi. You might have encountered these eels back in World 4-3, but you'll mainly find the smaller versions of Unagi in this section. These Unagi hide inside holes in the walls, lunging out when you get close.
As you start swimming upward, you will first move past three Unagi on the right wall. Hug the left wall until you move past the third Unagi. Move left and up away from another Unagi situated at the corner. As you move up, you will encounter a small Unagi that continuously moves across the screen from left to right. Swim behind them and up to find a red ring in an alcove a bit off the beaten path. Once you activate it, all but one of the red coins will be on your route upward. However, you must contend with three moving Unagi, and you move in the opposite direction to them, so its harder to swim past them. Continue upward, past some coins and another moving Unagi, and you will go through the checkpoint.
As you swim up the remainder of the channel, you will end up at a question block containing a power-up just below a one-way platform near a brick block wall. Once you go past the one-way platform, the wall breaks apart, revealing a menacing Mega Unagi behind it! The Mega Unagi fills up most of the screen, and it will pursue you to the end of the stage, so start swimming for your life toward the right. There are several pipes that push you to the right to help propel you forward. Watch out for large bubbles and smaller Unagi. They could get in your way or bump you right into the big Unagi.
Eventually, you should come upon a P-Switch on the ceiling. Hitting it will make tons of silver coins appear for the following section. Further on, after passing by a pipe, you can see the first Star Coin. Drop down to collect it, and start swimming upward if you are still tall Mario. You can break two brick blocks on the ceiling to get the second Star Coin shortly after the first. After swimming past some rows of blocks, you will reach a second red ring at the top of the screen. You must stay near the top for 5 of the red coins it spawns, then drop down to collect the remaining three.
Once you past the red coins, you probably won't need to worry about Unagi any more, only Cheep Cheeps. Keep adding more distance between you and the Mega Unagi as you approach the end. Past a reef pillar, you must make turn around while sinking down to collect the final Star Coin, although this maneuver may also let the Mega Unagi catch up. To the right are two pipes. You might be tempted to go into the green pipe to quickly escape the Mega Unagi, but if you swim a bit further and upward, you can enter the red pipe. Both lead to the flagpole zone, but the red pipe leaves you a bit higher up. If you emerge from the red pipe, you can jump on to the Paratroopa next to the flagpole to give yourself a boost to the top of the pole.
- During the Mega Unagi chase, after activating the P-Switch, start swimming close to the ground. The Star Coin is in the upper right corner of a reef formation, there for you to collect.
- After the first Star Coin, swim to the top and break the brick blocks covering the next Star Coin.
- This Star Coin is located near the end. You must quickly turn around past a wall and sink below the brick blocks for the coin.