I can see how exposed positive wires in a bilge could cause havoc INSIDE a boat but I fail to see how that would erode a zinc OUTSIDE the boat.
Thinking of scenarios, it could be that the shaft is insulated from the engine
so positive leakage inside the bilge could put positive potential on the shaft. But outside the boat, there would still have to be something else of significant size in the water that is connected to battery
negative to complete the circuit.
At least the first line of attack would be to make sure the shaft is grounded to the engine
(battery negative). If you have a non-metallic coupling you may need to mount a small jumper across it.
On a steel
boat it is not uncommon to see a grounding brush on the prop shaft since a flexible coupling plus the rubber lined cutless bearings can insulate the shaft.
But in general if the shaft is grounded, electrolysis
problems inside a bilge are independent from electrolysis problems external to the hull