Temporarily disable the gas burner or electrical heating element. Depending on your hot water heater, you may find a pilot switch that kills the flame and gas supply in one step. Put safety first:  read and follow the manufacturer's directions to ensure that you don'leave a gas valve open while you work. If your water heater is electric, it may have a disconnect switch at the tank or you may need to go to the breaker box and switch off the electrical circuit which feeds it.