First of all dive down here to find an open door. Take a right and go through another door to resurface. Climb up to the air vents and crawl along those now. Squeeze in between the pipes and the wall to find a wood pallet to throw down to Ellie. Hop down yourself, and swim Ellie across the water using the pallet She'll climb up and drop down a big ladder for you. It will fall off the wall, so carry it over to the gap you can't cross and lay it down. Do this by climbing back up to that ledge you were on, pick up the ladder, drop it over by the broken walkway, climb down, then lay it across the gap. Here is a quick video I made to show you how to do it enjoy. View video here.