Lara starts exploring a new building, but your main goal is to swim over to the other part of the flooded underground. This is a two-phase task, in the middle of which you will have to resurface to get some air and swim through the other part of the cave. Remember to tap rhythmically the B button or you will drown!
As soon as you resurface, walk around the ruins and zip down the line into the valley. During the cutscene, you will meet Jacob's daughter again.