And in fact, my boat is likely not correctly bonded, so I'm not pointing fingers. But your case I believe is a classic FOR bonding(stray current originating inside the boat). I just checked some bonding info on Wings Boatowners Electrical handbook. He cites 2 examples where bonding would have prevented stray current corrosion.

In one example, in a NOT bonded boat, he uses a bilge pump that is shorted to the housing. The pump would still operate(like your radar), because it's not shorted to a grounded conductor(back to DC-Neg). But stray current in his example gets carried through the bilge water to a nearby through-hull. The stray current (always looking for ground) goes out the through-hull and to the prop, because the shaft is connected to battery banks(via the engine block) NEG side.

Then Wing quotes,"If the pump housing had been electrically bonded, the short would have tripped the breaker, preventing corrosion(and alerting you)".

This makes me think your stray current, which was distributed through all the non current carrying water contact terminals connected, went looking for ground. It found it! It made ground through your prop shaft and back to your battery, and when it did, it caused the fast electrolysis.

This makes sense to me because the bonding system, if working correctly, is also a separate grounding system for any non current carrying components that are connected(like fuel tanks, windlass), as these are all carried back to the boats DC negative side as well as to the water outside the hull.

Does this scenario make any sense? I'm no expert of this stuff, just an eager student.