The commons are all connected together. Heres the senerio :

Boat A has a misconnected wire charging the boats system
neutral.............Boat A is plugged into the dock..........current from boat
A travels through neutral leg, which are tied together due to code and into
boat B shore cable..........hence into boat B......through the bonding
system..........through the water.........back to boat A. Zincs from boat B
are plated onto boat A.

Dave