[sammyg2]

Water isn't compressible, and there shouldn't be any air gap in a water heater, so he only time it could "rocket" is if the wa wa turns to steam.
The desire to expand would create thrust.

Seems to me that you'd have to screw up pretty bad to make steam.
Stuck open thermo-stat, failed pressure/temperature relieve valve, etc.

But murphy says is can happen.