Growing up in the north east, my parents had a solar pool heater (they still do), Heating a pool via thermal collectors meant that we could reasonably use the pool through the end of September / early October some years, and it wouldn't be freezing cold on hot days in June before the water had a chance to warm up naturally.

It also had the side effect of keeping the heat off our roof and would actively pull heat out of the attic. PV panels would have worked with a heat pump but probably would have cost a ton more for the required capacity needed. Not to mention the extra heat on the roof from not having it watercooled.