While I don't suggest this for most people in an urban or suburban environment, a wood fired heater could be used as well, if you have access to cheap fuel.
Living in a small, semi rural town, I'm able to use my home built heater to great effect.
We've had an unusual summer here. Normal August temperatures run in the high 90's. So far this year, we've seen that maybe for 3-5 days total. Temps have been running a fairly consistent mid 80's. Overnight lows have been running in the mid 60's.
Using nothing but scrap wood donated by others (just so they didn't have to pay dump fees) and cutting up old pallets from a local lumber yard, I've been able to keep my pool at 85 - 90, as long as I keep it covered at night.
What started out as an experiment and could use some improvement appears to be working out quite nicely.
It is a bit of a shock to me is getting out of a 86* pool to air temps in the mid 60's. Can you say very quick dry off and quicktime steps to the house?
Currently, I have 3/4" manifolds, feeding six 3/4" runners approximately 5' long each. fairly small firebox, which holds only enough wood for about an hour of burn time. Heat gain intake to output runs average 15*+. Better flow through the system would drop the intake/return temp difference, but increase overall heat gain in the pool.
Prime mover is a 1000 gph Intex pump without the filter.
Improvements planned:
Better heat exchanger, with 1" copper manifolds, with minimum of twelve 1/2" runners, increased in length to 8'.
Larger firebox, with better air control, for longer burn times.
Plump in a spare 2500 gph prime mover (pump), increasing the flow through the exchanger.
Just putting this out there for consideration, as wood heat in some cases can be more economical than natural gas or propane. Trade-off is a bit more work for the pool owner.