After exploring the TNT cave for the first time, move out onto the ledge overlooking the pool and turn right, toward the twin waterfalls. The screenshot above shows the path from here to the medi pack on the far side of the falls. Each step is detailed below.
First, jump onto the angled block on the right canyon wall, slide back, and grab the edge.
If Lara brings her feet up onto the rock ladder, momentarily release and re-press the Action button so she'll let go and grab the edge with her hands only. Traverse to the left until you can pull up on a nearly flat ledge.
Turn around to face the block between the two waterfalls. The side facing you is flat; the other side is angled. Take a standing jump from the edge of this ledge to grab the back of the angled block.
...slide forward just a little, and then jump onto the ledge ahead. Climb onto the next ledge and get the small medi pack. Then hop back down to the ledge where you landed when you slid and jumped.
Walk to the outer corner of the ledge nearest the waterfall. Take a standing jump from that corner, without pressing Action, to land on the little ledge inside the cave, between the two waterfalls.
Walk to the edge of this ledge and take a standing jump to the next ledge.
Take a running jump to land on the ledge jutting out at the next corner.
Then take a running jump across the stream to the ledge sticking out of the opposite cavern wall. Angle this jump so Lara takes off at the outer corner of the ledge, as shown above, and she'll have no trouble making it across. From there, head around the corner to the right.
- OR -
Alternatively, you can take a slightly longer route upstream from the ledge where you found the medi pack. This is shown in the screenshot above. Take a standing jump from the ledge below the one where you found the medi pack to grab the horizontal crevice. Traverse to the right, drop onto the angled block, and backflip across the water to land on the ledge behind. Then continue jumping across the ledges as shown in the previous screenshots.