Inexpensive Solar Pool Heaters Are Widely Available
If you Google "Solar Pool Heaters" you will see that there are many to choose from and most are inexpensive. Many have photovoltaic panels to power the pump that circulates water through the panel. There is no reason why anyone needs to heat an in-ground or above-ground pool with natural gas or electricity off the grid.
I'm writing my condo association and asking that it purchase a solar heater and install it ASAP to heat water before our pool opens in May.