Gather some resources from the pool of water near the camp, and then climb back out of the water.
Jump down into the water and exit to find the Collapsed Temple base camp.
Through the latter, you’ll enter a large room. One lets you interact with a breakable wall to create another shortcut. Swim on the bottom, and look for the survival cache. In the same area as the barrier you destroy with the shotgun, enter the water. (If you don’t have the shotgun, you need to the do the main mission that veers in the opposite direction of the challenge tomb.) Hop across the chasm, climb up the wall and use the shotgun to blast away a barrier and create a shortcut. Immediately after you rappel down, head through the narrow passageway. After you leave the water, there’s only one path ahead.
