[Jack Houpe]

I believe that's a series-wired switch; this is where all the thermostat does is interrupt the power (hot wire) to the termination point, which is your heater. The only time you would read a difference of potential is if you had a load on the other end where the heater is. If you just wire those two together, it should provide power to the heater. In some cases that switch will activate a relay that would provide power to the load via a low voltage source.


the plug for the water pump comes out of it and the heater plugs into it.

I'm not picturing this in my head. Both water pump and heater on the same plug?