I thought some people might find this story interesting.
Seattle has quite a few houseboats. These used to be very cheap ways to live, but now they are VERY expensive, since mooring space is limited.
They are houses built on floats, moored together on docks sticking out onto Seattle's Lake Union. You may have seen an example in the movie "Sleepless in Seattle."
I was doing repairs on a gas fireplace in one of these homes, and had occasion to discuss the heating system. It seems they used a heat pump, taking water from the lake under the house and returning the water to the lake.
Since it's VERY rare for ice to form on the lake, I'll bet this works pretty well.
I thought it was interesting.