One thing to consider is that you can not use the hot water heater thermostat. it is not a constantly on variable device. Thats an easy fix though.

run the plug wire directly to the heater element and use a router speed control to varry the wattage

most hot water heaters are 220 volts unless they are in mobile homes. Check the element itself to see the voltage it is rated at. If you are using a 120 volt supply then tou need a 120 volt rated element. for 220 volts a 220 volt element is needed.
applying 120 volts to a 220 volt element will only give 25% of its rated wattage. Be sure your router speed control is rated for more current than you will draw at maximum. be sure you router speed control is rated for the proper voltage. most folks use 110 volt supply and a 110 volt router speed control

I think that a dual element heater has the elements too far apart for versatility.

Newbie here but thought I'd compliment your great conceptual idea. I think (IMHO only) that this idea merits some research and someone actually doing it. A hot water heater is really set-up for safely using gas (propane also) produced heat INDOORS. That is, it is vented to the outdoors, has a drain hole built in, has the water inlet/outlet holes on top which can be used for temperature port a well as a means to introduce water directly inside without any mess and finally, has the electrode heating element port with available a myriad of wattages readily available. It really does seem like a no brainer. The majority of hot water heaters are glass lined thus the cleaning issues would be minimized however, the only thing I can see that needs to be addressed (beides controlling the electrode) is cleaning which has to factor size, weight and access limitations, but all these things could be worked out. Perhaps a mobile home or RV type would be better in that they are much smaller and many (the RV ones) are already set-up to operate on electricity (120 V) or propane. GREAT IDEA; got me thinkin'... THANKS

"Here is a post by JOHNB that tells the story quite well

The thermostat on your hotplate((( OR HOT WATER HEATER))) is most likely just a switch, not a power controller. So it literally switches power on and off based on the temperature surrounding the switch. All of that switching is going to affect the vaporization rate in your boiler.

If you're running a pot still it may not be a huge issue. If you're running a column it will cause the temperature in your column to move all over the place -- which is the opposite of what you want when you're trying to reach a steady state during reflux.

So if it prevents you from reaching equilibrium, refluxing and/or getting good separation (due to all of the temperature instability), then yes, bypassing the thermostat and using a RSC can lead to a better product."