[BB_TX]

I would look at how the loads are connected to your 120 vac load center. The only reason I can think that a shorted water heater would trip the main 50 amp breaker without tripping its own breaker is that if there were too many high amp items connected to one hot leg of the main breaker. If, for example, both A/Cs, the microwave, the converter, other things, AND the water heater were all on one leg, then high amps from the water heater could cause the main breaker to trip before its own breaker.

Of course, there is also the possibility the water heater breaker is bad and did not trip properly.