Everyone should have a solar cover. It’s inexpensive, easy to use, and it will pick you up four to eight additional weeks of swimming – that depends on how you use the pool. It conserves water by preventing evaporation.
Gas heaters are now available for around $2500. An excellent alternative for a smaller pool is an active solar heater that works off your existing filter. They’re around $2000 to $5000, and the heat’s free! Gas is faster, solar is very slow, particularly if it’s cloudy. Heat pumps are extremely energy-efficient. The only downside is that they require a bit more patience getting up to temperature in April or very early May. They may take week to do what gas can do in two days. But after that, they will maintain just as well for way less money. If you can handle opening your pool the week before you want to swim, that’s not a sacrifice. Oh, and if you cover your pool with a safety cover, you are going to end up opening it before it’s really hot out, anyway.