Your water heater has a thermostat that controls the water temperature. With no water pressure coming in to the tank you won't be able to get any hot water out of the tank, which ensures that there will always be water in it. The thermostat will maintain a constant hot water temperature whether or not you're able to use any hot water. You shouldn't need to shut off the power to the heater.

If you have room in your electrical panel for the addition of another 240 volt breaker, you really do need to have a dedicated breaker for each your heat pump and your hot water heater. If you have the room, it's a very simple proceedure to add the breaker. Any electrician can do it.