Electric water heater on water off 6 months will water heater be ok?
Not having fresh water flowing through it won't cause any particular harm since the water will deposit all of its salts/metals early on and just sit the rest of the time, so long as water stays in the system.  However the right thing to do since the dwelling will be unoccupied (the only reason it's allowable to disconnect water service) would be to turn off the unit (use the breaker or fuse panel to do this) since leaving it on risks that the water will drain somehow (overpressure valve opens, etc) resulting in a fire hazard. Even better, turn off and then drain the unit, which should be a simple process for modern installations since a drain is nearby (per code) and the water heater will have an easy to use drain spigot.