What would be a setting that could utilize naval travel?

I would really love to see more games in the series give us an open world that could really well utilize the naval explanation of past games. Not like in AC3 or Origins where you had isolated levels of Naval gameplay, but like Black Flag, Rogue, and Odyssey. Where you have the freedom to go anywhere on your boat and explore. But I genuinely can't think of any settings that could make up a good map for that sorta thing. Does anyone else have any ideas? (Not that it'll ever happen. I just like to brainstorm)