After exiting the cavern you'll find yourself outside and into the great outdoors. There's a zip line nearby, use it to get to the lower area. Then turn right and follow that path.
Jump on the zip line and use them to make your way across. You've been to this area before meeting with Grim.
Wait for a gondola to pass in front of you and jump on top of it. Ride it down to the lower area.
The gondola will then stop. Turn left and you'll see some rope wrapped on the foot of the tower. Fire a rope arrow on it and slide down.
Make your way to the back and you'll find a broken ladder. Jump on it and get to the top. You'll see another zip line there, use it to get to the ship. Also equip your machinegun before going down.
Hide behind the wooden panels and start shooting at their heads. There's not a lot of them so you won't have much problems.
After killing them a Solarii member will attempt to bring down the ship. Make your way up the yellow bars. Some of the bars will be revealed as you go up.
Once you get on stable ground run towards the other side. There's a yellow ladder there that you need to jump on. Don't stop for anything cause those old wooden panels will not protect you from the machinegun.
After the ship falls down you're going to go on the zip line. Once the screen slows down press shift to drop down on the next zip line.
After landing and sliding down on the path, shoot the barricade and use the pole to jump towards the next zip line. Now make your way down the river.
After getting to the river, head up towards the dock on the side and climb up on it. Follow this path and you'll find a small crack on the rock. Enter it to get to the beach.