I almost forgot the harbour in the fort itself^^
But that it also faces north strenghens my conclusion, that the mainland coast has to be nort of fort joy.
I know that Island harbours almost always get build into the direction of the mainland, if said mainland is near that island.
From the images of sentinel Island I would identify that biome more northern, at least in a temperate, and salty (wich explaines the desertish look) climate.
Fort Joy Island seems more tropical to me based on the vegetation and even the sand and water.
The sand has a warm and yellowish color.
The Water is pretty clear and light blue, which also indicates a tropical climate.
Carribean Islands are a good example.
