CW
I guess from post #38 you do know what you are doing, you just need to do it. If the shafts are stainless steel and the props are bronze, the prop will fizz if its shaft is not connected to an anode, as you say.
Assuming then that shafts(hence props), anodes and your electrical system common are bonded and connected to the marina supply earth, without an isolator, everything could fizz, by different amounts - it's a matter of luck (not strictly luck, of course, but practically so).
You should get a galvanic isolator and I suggest you get one with ac and dc leakage monitoring - to see if there is still current flowing. The reason for the monitor is that currents can be induced by effects, intentional or accidental, other than galvanic cells. Accidentally, most likely, by faults in other boats' electrics. Intentionally, as some vessels and marine installations are protected by active corrosion prevention systems (Not likely to be the case in a marina - unless you are near a naval dockyard? but everyone would have complained by now). Either can result in potential differences higher than some galvanic isolators will resist, and current still flowing through your suppy earth and so through your precious metal bits.
Cavitation is most likely to occur around the trailing edges and fast moving parts of the forward face of the propellor blades, where the water pressure is prone to fall sharply to create vapour bubbles. So, looking at the distribution of corrosion/erosion should give a good idea as to what the dominant mechanism was.
Good luck with the rest of the engine re-build and I hope you solve the puzzle of the failure.