Boat Key

Underground Cavern (Reservoir B1): hanging on the wall inside the shack on the east side of the main chamber. From the boat, go back into the mine, up the stairs on the left, break the wooden beam marked with yellow tape with the Knife and climb down the ladder to the shack.

Starts the motorboat at the East Jetty to cross to the West Jetty.

Hint: the file The Reservoir and Giant Fish ("leave the boat key in the shack in the mine").