Mike,
Your reply went up while I was composing. With a plastic water supply, I'll bet there is no good way to insure that this event can't occur again. Put in a new service with ground rod and other connections to earth and the return impedance to the xformer will still be high, by comparison to what a metal municipal water system / neighbor's neutral would provide. An event that floats the neutral will still result in the voltage swings.
Al
