A heat pump water heater inside the house would lower your air conditioning bill by pumping that heat into the hot water, but it takes up as much space as any other tank heater. In FL you can safely put it in a garage with no freeze-up risk, or out on a porch if you insulate the lines. In a garage or outdoors it will run very efficiently (more efficiently in summer than in winter), but won't give you the modest air conditioning benefit.

An electric on-demand water heater requires a dediicated 50A 230VAC circuit, which you may or may not have remaining capacity for on your line. It's a huge intermittent load, which is the least-friendly kind of load to put on the utility (and by extension, other ratepayers.) If the regulations change so that residential ratepayers are assessed "demand charges" based on the highest draw in any 15 minute period an electric tankless is an extreme liablity. (So far very few utilities assess residential demand charges, but many are considering it, particularly in areas where there are many rooftop solar installations.)

If you have a big soaker tub you might need an 80 gallon heat pump water heater, but most houses can do just fine with a 50. The 3rd generation GE GeoSpring has apparently addressed all of the early model issues, and is fairly attractively priced. Heat pump water heaters are even subsidized by the utility in many areas as a cheap way of lowering peak grid loads.