If you have a high efficiency water heater they usually have a blower and on off switch. For a while (months) when we left on vacation I would turn off the heater using the switch, and they upon our return forget to turn it back on until someone wanted to take a shower, then I would hear about my mistake… Anyway I have put a controllable outlet between the 120 volt plug and then set-up smart lighting controls so that whenever everyone leaves the outlet shuts off, and turns on whenever someone returns.
Been working good for the last year, saving some expense also, I am not really sure how much since we have been having kids move out also. The trick is to single 120 volts supply and the on off switch for the water heater.