Advance ratio relates to tip speed and far field velocity so is quite different to the velocity ratio I am referring to.  As stated before, the velocity ratio for an efficient prop is close to 1 otherwise it is not an efficient prop.

With regard to dragging the prop backwards, you are not thinking about a proa, which sails in both directions - or would regenerate in both directions if it had merit.  An efficient prop operates at very low angle of attack.  So even if your blades have a very fine trailing edge they still perform quite well in reverse rotation. Most props do not have a particularly fine trailing edge so work quite well in either direction under load. As far as I can detect the 18m proa motors in either direction with similar efficiency.  It is desirable to have the drives at the aft end if motoring in a decent sea because they get buried deeper when at the forward end of the boat; as occurs with any boat.  So, as a turbine, the blades could be working in the normal forward direction or the reverse direction.  Once spinning the turbine will have very low velocity ratio because you normally charge at a much lower current rate than you draw under load.  Typically LiFePO4 cells have a maximum sustained discharge at 0.5C and recommended maximum charge of 0.2C; meaning 5 hours to charge from flat.    

I can assure you that you would not want to try to stop a 540mm diameter, 3-blade turbine from spinning when doing 20kts through water.  It would slow insignificantly when charging at allowable charge rate if the controller allowed the high voltage required.

With regard to electrical flashovers - Water is ever present on a small boat.  You can buy submersible motors but they are very expensive compared with more open types - see the Krautner prices.  Even if the motor or generator is kept dry, you can easily cause flashover by overspeeding when completely dry.  It is a matter of whether you get an electrical failure or a mechanical failure first.

Rick