"the remains of the slip where the handles of the rusty winch"

GNU Free Documentation LicenseHandy-winch - Credit: Bela
A winch will pull in, release or adjust the tension of a rope. The winch's position close to the slip is indicative that it was probably located there to pull in a ship or boat perhaps belonging to previous residents of the island. The winch would have been similar to that in the image below.