it should have allowed the current, unrestricted, to flow from the short back to the breaker. It would have flipped(right?).

Because if the bonding is correct and connections are good, it also connects to your main DC panel ground/batteries, straight away, right?.

If the mast was not bonded correctly(or the connection in the bonding system to DC-neg is corroded?), it could flow to all connected bonded fittings, those of which underwater might pass current through the water to your stern tube perhaps, and then to DC neg in a stray current the breaker would handle? This would result in the fast corrosion to those underwater fittings?

Just thinking out loud, I gotta go.