Is it safe to unplug my electric water heater daily to save energy?
I would suggest a timer like this. Additional start/stop pins can be added if you decide on adding a second on off time (or more) and you can always flip the over ride to the On position. This way your water will be hot in the morning instead of having to wait after you turn it back on. 

The only problem is if there is a power outage you would need to adjust the clock face (very easy). I would suggest an absolute minimum rating of 30 amps. This model is rated at 40 and I have had excellent long term service with this brand and style.