What I'm guessing is that the darkness is everywhere (oceans, caves and all), with the exception of cities. They do everything in them, so if there's a city overlapping a lake or ocean, they'd fish there? Different towns for different departments is what I'm saying. Town dedicated to mining, one for wood, farming, tech, etc?
Painter said travel between cities is possible, so imports/exports would happen. Rarely, because of the fear surrounding the darkness.