When I drain my electric water heater for the winter, does all the water come out, or is there still some at the bottom? If so, is it ok if it freezes?
A little water will remain in the water heater, which is fine. It may freeze, but will not cause a problem.
Freezing water is a problem because of the pressure it creates when the water expands as it freezes. If the freezing water is in a sealed system or pipe, it will burst the pipe or push open the joints.
The water heater is not sealed so there should be no problem. I would leave the drain valve open over the winter.