Not an expert, but the much higher heat capacity (and heat conductivity) of water compared to air is likely one reason. But also, when it’s really cold outside

Not an expert, but the much higher heat capacity (and heat conductivity) of water compared to air is likely one reason. But also, when it’s really cold outside, an air to air heat pump is much less efficient. If you have access to liquid water, that is of course much warmer than outside air.