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Electric water heaters are insulated pretty well, so you probably won't save much by turning it off. 100% of the energy goes into the water as heat (or very nearly so), so the only losses you can save are "standby losses" from heat radiating out of the jacket. Because electric water heaters don't have a flue that tends to cool the water, they usually retain the heat very well.
If you turn it off after it is hot, leave it off for 8 hours and turn it back on, does the element have to come on to heat the water again? If it does, spend your money on insulating the tank, the pipes coming out of the tank, installing a "thermal trap" in the outlet pipe, etc. If the water in the tank is still above room temperature then you haven't lost all of the heat and you won't save much energy by waiting until you come home to turn the elements back on. The intermittent use of the elements during the day will come very, very close to the longer run you need when you turn the elements back on.
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