Well, I asked my brother about this. because he's taken some airconditioning/heat pump, freon classes, and got some sort of basic certification in that stuff. Not a seasoned veteran of the HVAC business, but a bright guy who wanted to learn more about this to help with maintenance of his various properties.
Anyhow, he says a plain old heat pump, if its heat exchanger were immersed in ground water, or any other liquid medium of moderate temperature, would work just fine. After all, the freon or other working fluid inside the heat exchanger does not know or care what's on the other side of the metal membrane that it exchanges heat with.
So, why not put your heat pump down the water well, or whatever?