If a water heater is electric, the circuit breaker switch that feeds electricity to that circuit turns the unit off, and if it is gas, turning the thermostat dial to Off shuts the power off. This knowledge is crucial when the water heater starts to make menacing noises.
Turning the water heater off is also important for draining purposes, which should take place every three months to avoid sediment aggregation. Turning off the water supply is also necessary for draining. The water valve on the supply line to the heater stops sending water if it is turned clockwise until its stopping point. The main water valve for the house is a backup shutoff.