Heat pump HW heaters work GREAT in hot garages from an efficiency point of view, but don't expect the cooling effect to be noticeable. The thing to watch out for is product reliabilty/warranty issues. IIRC there were some reliability issues with the offshore manufactured GEs, but not the US versions of the same. I haven't heard/read much either way on the Rheem units.
Don't expect to notice much in your electric bill either, given that it's a 1-person household most of the time. It will cut your HW heating power use by more than half, but half of "small" is still small.