[diggernet]

It does exist, and those plumbers are wrong. The biggest challenge is finding a heat exchanger (to replace the one in your AC condenser) which is designed to survive highly corrosive hot swimming pool water.

The UCDavis Western Cooling Efficiency Center [1] has done extensive research on this, sponsored by the California Energy Commission.

In a nutshell, they found that using your swimming pool as a heat sink for your air conditioner can reduce cooling energy consumption by 30% or more (not including the reduction in pool heating energy consumption). The two main caveats:

1. Many pools are too small relative to the house to be an effective heat sink without overheating the pool.

2. Increased pool temperature causes increased evaporation, resulting in increased water consumption (also a problem in CA).

They also found that adding a fountain for evaporative cooling of the pool can mitigate #1, at the expense of worsening #2.

[1]https://wcec.ucdavis.edu/