The meter looks good and I've seen my meter move just as fast. One thing about an electric water heater, nearly 100% of the input energy is converted to heat. It's not costing more to heat the water unless there is a run away thermostat but then your water would be scalding hot.
If you want to check the water heater and you're not afraid working with electricity here are few tips. YouTube has hundreds of videos on testing electric WH's.
You'll need a clamp meter to check out your electric water heater. Each element is usually 4500 watts at 240 volts. That's is just over 18 amps current draw. If your upper or lower thermostats was bad that keeps both heating elements on, the current draw would be over 30 amps and your 30 amp breaker would trip. If your line voltage is 220v, current will be lower. Only one element is on at one time.
A clamp meter is also a volt ohm meter. To check the element disconnect the power. Then remove at least one wire at the heating element and measure the ohm across the terminals. For a 4500 watt element it will read around 13 ohms.
If your good at it, remove the breaker panel, with the WH heater on, place the clamp around one of the hot wires going to the WH. It'll read about 18 amps, less if the line voltage is less than 240v.
