|Lost Copper Isle|
The map turned into some sort of fantasy version of El Fraile Island, a sea fort carved out of a rock outcropping. Now the way I figure it the fort is relatively new and recently occupied by pirates who made a few improvements before being driven off. Prior to that the whole island (let's call it "lost copper island" was some of temple complex or ritual site for a sea deity and its best if it's some kind of icky Cthulhu style sea deity I think. The abandoned pirate fort fills only a third of the space with set of sea caves, a flooded shrine and perhaps some crypts filling out the island. There's even a little cave for a creepy old hermit in a battered coracle who might be able to impress upon characters the danger or potential glory of the island.