Buck is not too far from your current position, so head over to the marker to start this mission. Once Buck's history lesson is over, head through the door ahead of you. You're now inside of a bunker that's pretty wrecked. Turn right and look at the Japanese lettering on the wall, and you'll see some rope above it, so climb up. Find the next rope to climb up to the next section.
There will be some stairs leading down, and there's a snake in the far corner of the room. Go down the stairs and drop into the water below. Swim through the pipe and you'll be in a nice big area, where you'll find some enemies. If you have the Death From Below skill, swim up to the enemy by where you can climb up and take him down.
There's another guy above you, in the same situation as the previous one, one ahead of him, one above on the platform at the far end, and one more on the boat to your right. After killing them all, go through the tunnel on your left until you can drop down into a new area. There will be about 5-6 enemies here, so take them all down and survey the area.
There's a stone door ahead with some dynamite attached to it. Go to the detonator and use it to blast it open, creating a hole for you to go through. Follow the path to a spot where you have to go underwater, but you'll be attacked by a crocodile as soon as you go in the water. Kill it, then go through the water and out on the other side.
In this new room, go up to the door and press the seal to open it, then drop down into the water and come out on the other side. Continue into the big open room, go down the path and jump over the gap, only to have the rocks crumble, with you going down into the water below. Swim up to the opening and you'll be on the bottom of the previous room.
You'll need to solve the puzzle to get to the treasure room, so start by going to the seal door on the right and pressing it. Inside this room, break the three flashing pieces of wood by slashing them with your knife, which will raise the water level. Go back to the previous room and swim through the hole in the wall on the opposite side, breaking the three pieces of wood in this room to raise the water again.
When you try to exit the room, a boulder will drop down and block it. Instead of going through it, swim to the top and go over the blocked path and back to the previous room. The water level is now high enough where you can climb and enter the treasure room. Grab the item on the altar, which will make boulders fall and block the exit. Go to the ledge and there's a spot on the left side where you can shimmy across. This path will lead you outside, where you'll find Buck waiting for you, as well as the end of the mission.
