Maybe the impressed current takes the line of least resistance,ie through the Sea when in Salt Water but when in Fresh it finds that the path of least resistance is through your electrical control systems?
As you say the ICPP won,t be doing much in Fresh Water, I would check the makers manual, the ICPP systems should certainly be shut off when you are alongside another steel structure and possibly in Fresh Water too.
It is well worth shutting of the ICPP when in Fresh water to see if that solves the problem.
Without any corrosion protection you have a big "Battery" formed by the steel Hull and Bronze Propeller, the ICPP should bring the steel Hull potential Volts to the same as the Bronze Prop and stop the flow of current between them which deposits steel ions on the surface of the prop, where they are promptly washed away. It may also have links to the shaft Earthing, you may have nearly cracked this one on your own,
In Salt Water you use Zinc sacrificial anodes on steel Hulls but in Fresh water they don't work and you use Magnesium sacrificial anodes, because the metals react differently with each other in Fresh and Salt Water.
Good luck with the problem solving and have a good leave.
It is always better to ask a stupid question than to do a stupid thing.