You missed one of the most important and possibly hard-to-attain pre-requisites, Tom.  Lack of tide and steady flow both matter to the power. But even more important is that traffic up and downstream at the point of the wire has to be minimal, and all able to fit beneath the wire going across, including masts and dishes.  Put all of those requirements together and you have an extremely situational design best suited for places without much water traffic, and thus without much demand for a dedicated ferry at all.