Access the Crypt
From the Kuwaq Yaku Base Camp, cross the bridge to the north and hop into the river to the left. Swim toward the beach on the opposite shore. Cross the bank and dive into the small cave. Swim underwater through the tunnel until you can exit to the left.
After you climb up the ledge and drop back down, a side path leads back to the water you just exited. Search that route now, or on your way out. Back on the main path, climb up the other side to find the crypt entrance barricaded. Pick up the bottle next to the barricade, craft a Molotov cocktail, and use it to burn the debris to the ground.
Find the Sarcophagus
Wind your way through the first rooms of the crypt and down a corridor. The floor gives way, sending Lara sliding down the gradient. Dodge right, left, and right again to avoid the deadly spikes. Leap over the pit, throw the Grapple Axe, and swing to the far ledge. Grab on if necessary.
Continue forward, over a gap in the floor, until you reach the sarcophagus. Reap your reward before returning the way you came.
Exit the Crypt
After swinging from the hook, attach to the far wall and follow the craggy surface right, then up. Climb and scramble all the way up to the ledge above. Stick to this upper path, using the protrusion to clear the next gap, and wall-scramble to reach the ledge.
Walk out onto the stone block, jump toward another hook, and swing into the corridor. This sends you just past the point where the floor gave way as you entered the crypt. Exit the cave the way you came in.
Access the Crypt
Accessing this crypt requires a shotgun, so return after completing the Cenotes region. From the Hunting Grounds Base Camp, follow the water to the right. At the far end, pull out your shotgun and destroy the barrier on the left.
Find the Sarcophagus
The treasure sits inside the one chamber crypt. Loot the sarcophagus and return outside.