It is, since the obvious alternative to taking the heat from water would be taking the heat from the air or from the ground.

The air is colder in winter than the water, and the ground only provides a limited amount of heat before you can't extract any more. So water beats both.

It is a bit relevant because if the air was warm enough you would be better building huge air source heat pumps.

And if it was really warm enough you wouldn't need heating in the first place.