When I first read this, I wondered how a GFCI would have helped if there had been a fault to an ungrounded case. But if the energized conductor faulted to the case and there was no return path for the current, it would have raised the water heater and the water to 120V. The fact that the water elevated to a lower voltage indicates that there was a high impedance path available. Since there was another path available, the current on the phase conductor and that on the neutral conductor would not have been equal. That imbalance would have been enough to cause a GFCI to operate. However, if there was no path whatsoever (like some of the older installations), a GFCI would not have helped. GFCIs are wonderful things, but they won't work as desired if there is nothing for them to sense. Remember to check your system to make sure that it will work properly and that you aren't just ending up installing a false sense of security.