and others have a great deal of information available. I have had many happy solar heat customers since the 1970's energy heydey. The usual error is to think that something that looks like a solar heater is the important part. The first idea is to put a coil of black poly on the roof. Each square foot of actual surface exposed to the sun will heat a gallon of water per day about 10 degrees. There are many gallons of water in a pool, so there will need to be many square feet of surface. This would take many coils of plastic tubing at high expense.
A less expensive and useful panel system would use a spray of water on black panels on almost the entire side of the roof toward the sun, as in
allowing the water to run down to a collection gutter to go back to the pool.