Water Heater Blowing Fuses

To the best of my knowledge there are no 12V heater elements in water heaters. If the 12V fuse is blowing it would be a short in the system someplace between the switch to turn it on and the igniter in the heater.

I would say it doesn't matter whether 12 or 120 volts. The element is possibly shorted to ground or through the water to ground. With electricity off (all of it, 12 and 120) take at least one wire off the element at the water heater and test for continuity from the element screw to the other one, then to the side of the tank. If there is a beep from one side of the element to the other, the element is good. If the beep is from one side to ground (the tank) it is shorted and this will pop a breaker. A partially open element where electric current is exposed to water will also sometimes cause a fuse or breaker to blow. hth

Need a little more info. Are you blowing a 12V fuse or a 110v breaker.