Meet up with Buck then examine the bulk door to enter the abandoned japanese bunnker. Explore the area until you reach the submarine pen. Clear the enemies there then continue to the cave entrance. You'll find more enemies there so clear them out and operate the remote to trigger the explosives.
Next, explore the chinese ruins. Jump to the water and kill the crocodile that's lurking there. Head to the next entrance. Continue following the path then jump across the gap. You'll fall to the water below.
Next, activate the eastern switch then shoot the rotten support beams to fill the area with some water. Activate the second switch and swim over it and destroy the other support beams as well. Once the area is filled with more water, find your way out then swim to the treasure room.
Head outside and pick up the compass piece. Find the exit to leave the area. After meeting up with Keith, the mission will be complete.