Your water heater will continue to operate whether the water is turned off or not. If its electric its a problem because eventually the upper element will become exposed to air and burn out. Gas doesn't matter so much since it would have to evaporate 40-50 gallons of water.. But if its electric just turn off the breaker, or if its gas you can either turn the gas valve to "Vacation" mode which just keeps the pilot lit or shut off the gas supply altogether to be extra safe.