Having dived into the water from his exploding helicopter, Golgo 13 must swim to the next enemy base and rescue Cherry Grace. The layout of the water is as big as the one from Act 3, but the layout is much simpler. The multiple pathways have been removed, so you are forced to swim through the entire length of it in order to reach your destination. You won't discover any new threats while you are swimming, just an abundance of the ones you've already seen. Plenty of sharks will attack in the highest level, and octopuses float about in the remaining two. However, when you are ambushed, you will encounter submarines that join the fight along with the divers. They are dangerous, but they attack less frequently than the divers themselves, so you can save them for last.
After you swim all they way from one side to the other in each of the three levels of water, you will reach the access hatch to the interior. Inside, you will deal with the usual enemies: heat seeking missiles, laser defense systems, and officers riding floating platforms. Avoid what you can, and fight only what you have to. When you make it to the right side, you will encounter a man claiming to Oz Windham. Evidently the man you've been dealing with was not the real Oz Windham, but was in fact the leader of Drek in disguise. Furthermore, Eve Christy is the leader's girlfriend. When Oz tells you that the enemy has fled, Golgo 13 recalls the sea plane he saw escaping during an ambush. Oz informs you that Cherry should be found nearby. You will then be placed inside the next enemy base.
Inside the base
You start out at location 2 on the first floor. After tossing a grenade out to blow up a hole in the first wall, you must make your way to the north side of the building in order to reach location 3. This is where you will find Cherry Grace being held captive. Upon rescuing her, she will give you a key and tell you to locate a file that will tell you who the enemy is. Then she is off to Rio.
The file is located in the lower left corner of the building, marked by the red "a" on the map. Along the way, you should try to avoid the pitfalls, like the one marked with a yellow "D". However, if you do find yourself falling to the basement, there are two items that you can collect. The spot marked with a red "b" contains a stockpile of ammunition, while the red "c" marks the location of a vial that will restore all of your health. To return to the first floor, you must access the long corridor along the south of the basement with three grenades. You'll have to use every one of them to blast through the three walls that try to keep you from reach the stairs to the first floor.
Once you have obtained the file, and read the message it contains, you should make your way to the exit, which is found at location 4. Remember that in order to reach it, you must avoid pitfall "C" by passing back through the first door, walking north, and passing through to the second. If you do not have the file, the game will not allow you to escape. Once you're through, you will be on your way to Rio Di Janeiro.