If you are electrically inclined, you could wire a household t-stat in series with your heater and set the temp at the highest point you ever wanted to see a temp, like 82. The thermostat has its own thermometer that could be dropped into the water. It would be just like turning on your furnace until it got to 82, then it simply won't let it work until it goes below 82 again. You would have a redundant temperature sensor/cutout.
Honewell makes one for baseboard electric heaters that could be modified (add a plug, maybe lengthen the thermometer/sensor wires to get it into the tank) -- model T498 --around $32.95. I'm sure there are others. Just an idea.
p.s. You might find something on the DIY page.