Pool water is corrosive to most metals, you need to have the right heat ex. I have a new one designed for a Desert Aire unit. It is a tube in tube design made of copper. I dont cover my pool at night so I never have trouble overheating my pool, so I dont expect to have any trouble. If I do, I will put a pressure switch in and cycle the condenser fan On, to remove any excess heat that cant be removed by the pool water. It would probably be best to just install it that way to begin with. The easiest way to do it(in my opinion) would be to cut a tee into the hot gas line before the condenser coil, run the hot gas line to the heat exchanger and back through the condenser coil. This way you would be covered in case the pool water was too warm to use and the pressure switch would turn the condenser fan on.
If your pool pump and filter are close to the a/c it would obviously save a lot of work. My pool heater is 250,000 btu, my a/c is 5 ton (60,000btu). This would increase the effeciency of the a/c and give you free heat in the pool. These systems are used all over in bigger pools, why not a small residential application?
A stainless plate heat ex would work great also.