put one side of your tester one side of the thermostat and the otherside of your tester on the other side of thermostat, do not go to ground
The problem is the wire from your fuse box to the water heater is bad. You have 240 at breaker and you do not have 240 at heater, with power being disconnected at heater.
If you have the wires disconnected at the breaker, and breaker on, with around 240volts coming out of the breaker . The breaker is good. You are sending 240 out of the breaker it just not 240 volts at the heater. Is the water heater grouned? Do you live in .a house or trailer