Some people on this forum have recomendeded a system that channels the warm exhaust from AC to the water heater. In certain climates like in Florida, this works fine because it does two things: it provides free or nearly free hot water because it uses exhaust from the AC that would otherwise be dumped into the open air and wasted; it increases the efficiency of the AC. So it's a win/win.

But using such a system to heat a pool may not work so well. You really need to crunch some numbers -- maybe somebody on this forum can guide you. The thing is -- how much heat does your AC throw off versus the quantity and temperature of the swimming pool water and how many degrees you want to raise that temperature.

BTW, these are not geothermal systems, which involve underground tubing where the temperature is constant year-round. That's a different thing all together. When properly designed and installed they can greatly reduce the cost of heating, even in frigid climates like Canada, and they can reduce the cost of AC as well. But they are expensive and, if not done right, they won't save you very much money.