Sounds like your AC thermostat is malfunctioning. It should shut off via the high limit bellows and turn on via the low limit bellows.
I think your high limit bellows has a hole in it and is not opening to remove power from the heating element.
Assuming your water heater is a Suburban:
Replace the one you have with: Suburban Water Heater 130 Degree Eco Thermostat 232306 | eBay
It is an easy repair (one nut and two wires); just make sure to open the AC breaker in the power panel.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW