Also, you can’t mark it explored with a crew like James Cook and dock a ship at it. You have to have made an actual explore action.
I’m not sure why that wouldn’t work. The Rules/Code says:
• You must have a ship docked at a wild island you have explored in order to build a fort there.
–One of your ships must have explored the island.
Code on S-Exploring:
-This ability does not provide an explore action and does not allow you to look at any treasure on the island. It only allows the placement of an “explored” marker, which will allow your ships to explore as a free action when they dock at the island.
Once they dock there it is explored as a free action, meaning it has been explored before the end of turn when the fort would be built.
I’m not sure why that wouldn’t work; hopefully Woelf will chime in on this one too.