Ride the zip line down to the building.
If you let go just before reaching the doorway, Lara should land there without losing any health.
Climb down near the big chain.
Then climb up the opposite wall with the carved faces.
A poison-breathing lizard emerges from the dark hallway on the right. Shoot it.
Then go around to the left and use the switch to pull the plug in the pool below.
Follow the dark passage away from the chain.
The torch at the end of the hall ignites, and another lizard climbs up out of the opening on the right. You can back up as you shoot, but watch out for the hole in the floor near the chain.
Drop into the hole the lizard came from and slide down into the room with the kayak. If you didn't shoot the crocodile in the pool earlier, do it now. Then climb into the kayak and paddle downstream to the CAVERN WITH THE CHAIN AND PLUG.
When you used the switch in the room above, you raised the stone stopper out of the pool. The water is now swirling down the huge plug hole. Make sure Lara has full health as you approach it.
Back paddle a little to break your fall as the whirlpool carries you down the drain into the room below.
Climb out of the boat and either harpoon the crocodile...
...or swim to one of the ledges, climb out of the water, and shoot it from above.
Swim under the ledge with the wooden door and pull the underwater lever to open the door. This also lets in 2 more crocodiles. You can kill them or just avoid them and exit the level.