what if !
Yes, it is an interesting discussion; I''m glad if I am contributing.
There''s no getting around the laws of physics, so whatever propulsive method you use will still have to produce a net power equal to that required to move the hull through the water at the given speed. Some methods will be be more efficient than others, but the lowest power requirement you could ever have would never get lower than the sum of all the "drag and windage losses" (and that would require 100% efficiency).
As to using the prop to spin the motor as a generator and recharge the batteries, it was mentioned in an earlier post that such a thing is being done. Again, one must realize that in order for you to create a certain amount of electrical power, the mechanical power input to the generator must be larger (to account for losses). The mechanical power in this case comes from the waterflow over the propeller (which is not likely to be optimized for this purpose), and the amount of power losses due to drag force produced by the "water turbine" has to be even greater yet.
My interpretation of this is that the propeller/generator could not produce more than some fraction of the power it consumes while running, and it would place a heavy drag on the boat. For example, you run the motor off the batteries for one hour at 6 knots consuming 30 amp-hours of energy. At that point you switch the motor to generator mode and set sail for an hour. I doubt you could get much more than 15-amp hours of electrical energy back into your batteries and you would need a very decent breeze to move the boat above a couple of knots. I realize I am theorizing without hard data here (always dangerous), but I believe it would be similar to trying to sail with your gas outboard motor at medium speed in reverse.
Someone reading this might say that towed water turbines are already used by many. True, but they only need to provide enough electrical power for "normal" usage, not for a huge bank of batteries used to power the boat.
You are right about Edison; he was quite a driven man. It is good there are people out there who don''t give up so easily.