Yes HuwF, lots of folks have tried. The info above is all basically true, but folks are trying hard not to rain on your parade. As a practical device, it has absolutely no chance of replacing any generator. Turbines and propellers are different animals. Yes, they both have one moving part with one degree of freedom, so how hard can it be?
The trouble comes from the fluid, not the machinery. If you insist on thinking of it as a prop in reverse, you won't ever get a handle on it, but perhaps this analogy will help-
If the prop is moving the boat at 3 knots, the water jet created by the prop is probably three times this. So that is the speed you would have to tow at to get that sort of power. (Except that doesn't work either, it's actually somewhat worse than this if you are measuring power in/out of the batts). A solar panel is much easier to live with. I can cruise a 38'er on two panels and a 30amp controller feeding three WalMart batteries. (and no drag)
Fast Fred's comment about drag was probably intended to discourage you a bit. Mathematically, there isn't any natural relationship between the thrust made and the drag caused by the two cases. It depends on engineering, the drag could be nearly anything. Drag of turbines is usually a secondary issue for turbines. You won't find much info on it. A design for best power/drag will be different than a high nominal efficiency design. I have yet to see a report of any trolling motor producing 2% of its rated amperage behind a smallish sailboat.(peak,yes- average over time,no) I think some of the 20knot Vendee boats used a custom generator to manage the autohelm power draw. Or was it the ORMA boats? Really fast boats.
Some practial considerations. Propulsion benefits from the slowed wake behind a boat. Obviously, generation would suffer if towed in the wake.
The slipstream a prop creates effectively straightens out the inflow condition seen by the prop. The heave of the boat has less effect than you would expect. On a generator, the situation is reversed, any heave or inflow irregularity is magnified. Just watch any air generator on a pole on the transom of a sailboat. They don't work very well when the boat is underway in a breeze, which is real annoying to people (they also don't work very well when underway and there isn't a breeze
There's a bunch of info at the electric boat forums. Yahoo Groups has one.