I forgot the challenges’ names but here are these exit keys’ locations:
Long tail boat
It’s in a shack near the boat exit. Inside the shack you should find a brick on a surface. Pick up the brick and the key is underneath it.
Militia boat
It’s near the satellite device in the cave, on a table with the camera recording machine. A guard sits nearby, and picking it up is illegal.
Submarine
At top of a crate located in the ruins tower. This is where you meet Akka as the hippie.
Then here’s the answer you’re looking for: