Hi
If you don't want to go down the cost of a heat pump could you not pull your water from the ground and run it through a solar water panel on the roof of your shed. Even in winter you will get some gain, and if your into electronics a simple diff op amp could be used to control a solenoid and bi pass the panel if it is not hot enough.
One thing I haven't heard so far is COP factor with a heat pump, and for a low temperature gain for comfort heating you could expect a COP factor of at least 3:1 and possibly 5:1 or even 7:1 I have seen swimming pools with higher than that quoted but never seen it confirmed.
In case your not conversant with COP factor it is coefficient of performance, for every kilowatt of electricity you put into the thing you get 3 or more kilowatts or more of heat out.
Pity your swamp is so far away that would make an ideal heat source, freeze a great chunk of it solid and strip the latent energy as it tries to thaw out. When I worked in AC I wanted to try it with my parents swimming pool..... Bad suggestion my mother went on for weeks......
Brian