Apparently, sovereignty over Machias Seal Island is disputed between the US and Canada. Apparently it’s supposedly claimed as part of the State of Maine as well as part of the Province of New Brunswick.
What happens if you try to travel there from the US or Canada? Do you have to pass the other country’s Customs when docking there?
E.g. if I cross the border into Canada at a regular checkpoint, then hop on down to the Bay of Fundy and catch a boat to Machias Seal Island, is the US going to demand that I re-process into the US? If I wanted to go back to the mainland of Canada, would I need to pass Canadian customs again?
If you approach the island from a third country (e.g. you sailed all the way from Ireland up to the border between New England and the Maritimes and attempt to dock at Machias Seal Island, are you going to have to go through two customs checks (the US because the US claims it, and Canada because Canada claims it)?
The only thing on the island is a Canadian lighthouse, which is manned chiefly to underscore the fact that Canada claims it’s their island. A limited number of birding tours visit the island from Maine. I doubt that the two lighthouse keepers check the credentials of the tourists visiting to look at the puffins, and there’s no American personnel to check any Canadian traffic. Since the island has no recognized private claims, and is government property, according to either government, you probably can’t visit there legally without permission anyway, customs aside.