Yeah, there are of course also water-air, water-water pumps (although I've never heard of the open-circuit ones, but yeah, why not). But you have to have access

Yeah, there are of course also water-air, water-water pumps (although I've never heard of the open-circuit ones, but yeah, why not). But you have to have access to a large enough body of water that doesn't freeze and has enough capacity. The main issue for me with air-X heat pumps is that over time it can start to make a lot of noise if not well maintained. But the closed-circuit under-ground (or under-water) ones essentially don't need maintenance and are incredibly efficient, as the source never goes very cold.