Water heater sitting idle - problem?
First turn off the dedicated breaker in your panel to that water heater. (It probably is a 30 amp double breaker if it's a 220v water heater.)
Then turn the water supply off to it (should have a cold water supply cut-off valve near top of heater), open the nearby vanity sink hot water faucet and the T&P valve on the water heater for air, and gravity-drain it down with a water hose attached to the drain at the bottom.
That will take the power off and water pressure out of the heater. If you ever need it again, you can flush it out with water, fill up the tank, and then turn the power back on.