When you emerge on the ledge above Shipwreck Beach, turn left and move to the edge of the cliff. Stand near the anchor post and shoot a rope arrow into the rope-wrapped beam below. Slide down to the ledge near the fire barrel. Then move down the path a bit...
...and take out the 2 enemies on the walkway ahead.
Go down there, loot the bodies and pick up boxes of shotgun shells and rifle/SMG ammo, one at each end of the walkway.
Move out onto the painted wooden ledge and jump across the gap to land on the ledge below, where there's more shotgun and rifle ammo. Take what you need. Then drop down onto the next ledge.
Ride the zip line up to the tower overlooking the dock, where Lara's friends are engaged in a firefight with the Solarii.
Slide down the next rope line to the dock, and approach Reyes to initiate another cut scene.