With the group finally complete, go all the way back until you exit the tunnel. You will now have to return to the swamp. It's a long way off, but thanks to your new ally Gil you'll be able to take advantage of new underwater shortcuts that will make getting around much faster, as well as collecting new items along the way. Go back to the pond with the waterfall and the hanging platform you crossed earlier. If you need to take a Medicine, switch to Gil from the sub-screen. Enter the water and go down the tunnel at the bottom right. You'll come to a flooded underground room, with a swimming lightbulb-like monster and a chest containing a Medicine (if you don't need it, you can come back for it later). Then go up to the lake and drop into the precipice on the right.
Continue to the right to enter the area where you started the game. Go past this area until you reach the pond with the red mobile platform. Enter the lake using Gil and drop down into the opening at the bottom right. You will end up in an underground lake. Reach the left bank and get out of the water to find the first Life Container just beyond. If you have a Medicine in your inventory, it is advisable to use it right away, to fill the Life Container and free up space for another. Enter the lake again and go down further. In the room below, take the tunnel to the right. Continue to the right (you will notice a familiar tube along the way) until you have the opportunity to climb up. In the room above, take the tunnel to the right. You will then reach the lake at the bottom of the swamp.
Once here, go up to the top of the tree to the right to meet your bird friend and ask him for a ride. The bird will take you to the rocky area on the right. This time though, you can climb the wall to the right using Bop-Louie's suction cup. Proceed to the right and you will find a path that goes underground, with a chest next to it. The chest contains a Medicine, take it if you need it. Then take the path that goes down. Keep going down, killing the slimes and eventually you'll come right behind the tap you may have seen earlier. Jump on it to open the water and flood the area below.
Switch to Gil and jump into the water, taking the narrow tunnel just above the door. Then follow the path underwater. When you get to a point where you can get out of the water, do it, avoiding the duck (the path that goes down is a dead end). Proceed to the left and jump back into the water. At the bottom, take the path to the right and at the end you will find a chest with the Green Ring, that will allow you to see the position of the key of the same color on the map. Go back and this time take the path to the left. Go down when you can (going up you will be blocked by a brick wall) and keep following the path. Eventually you will meet a boss.
This boss is much tougher than the ones you have previously encountered. In fact, the fight takes place underwater and you will have to use the slow and clumsy Gil, while the boss will come swimming towards you. The strategy is still the same: jump on the boss's head to make it spit a ball, pick it up and throw it at it. In order not to be hit by the boss, you will have to calculate the times well. The best way to hit the boss is to swim clockwise around the room, jumping on its head when you are at the top point. Then, go under him, pick up the ball and throw it at the boss, bouncing it against the wall. On the fifth hit you will eliminate it.
Proceeding to the right, you will find a chest containing Gil's special weapon, which will allow you to destroy certain brick walls, which you have surely already noticed.