Is a small water heater an acceptable solution for extra heat on a heat pump system?

All very neat and good looking…

BUT

May I put in a word on behalf of the sparkies a few years down the road? Have a complete — and I do mean complete — notebook on your electrical system. A full wiring diagram. A ladder diagram. A description of where every device is. The original installation and commissioning instructions for every device. I know that sounds like overkill, but… I have worked on houses with non-standard wiring, and in the absence of such documentation your future sparky is going to have to stop and create it for himself or herself — on your dime.