Why douuble pole switch must be used for water heater?
Most water heaters are 240v, this voltage is created by using 2 hot legs, for a local disconnect or breaker both hots need to be disconnected. If a single pole was used as the local disconnect the water heater would not heat but there would still be 120v at the control to the elements and this creates a hazard to someone working on the water heater (like when time to change the elements). If it is a 120v water heater a single pole is acceptable (these are usually smaller under sink type where only a small amount of water is needed at a remote location from the main water heater).