I would add that it doesn't seem like it'd be worth-while if you're trying to get anything more than a little bit of luke-warm water- remember that the efficiency of a heat pump varies inversely with the temperature differential.
ie, you can expect much better performance if the ground is at 20'c and the water only has to be 30'c, than if the ground is 15'c and the desired water temperature is 50'c.
into the bargain, the higher the output temperature, the faster your compressor will wear.
for the same electrical input power, I'd expect more hot water from a pump and some solar collectors; of course, if you're dealing with home-owners limitations or other appearance concerns. . .