Make sure you have activated the mechanism that opens the waterfalls. Facing the mechanism, swim to the right, where you will find a ledge to climb out. Then climb on the next three stone blocks.

Climb up them, until you reach the third block. You will notice a huge brown rock that blocks your way. There is another (4th) ledge behind the huge rock that you are not supposed to reach, because the stone is in the way. The truth is that you CAN reach it, but the jump is quite tricky and many people have encountered problems. You need to jump off the ledge and pass just next to the rock. If everything goes well, Lara will grab the ledge behind.