I can't comprehend the harbor rules. We can't figure out if a manned harbor is needed to place a fishery. For example, there is a free (land) hex next to your city. Can you put a fishery there, abiding by the rules that both fishery hexes are adjacent to a lake?
Harbors only extend your influence - like Inns or other cities.
Fisheries allow you to "harvest" lakes. As long as your fishery is in your influence area already, you do not need a harbor.