On Fri, 24 May 2013 07:29:53 -0700 (PDT), "
wrote:

To try to roll your own is not a trivial task. And the
one system available, that was shown on This Old House,
requires cutting refrigerant lines to install the additional
water based heat exchanger, modifying the existing
system, recharging it, etc. All that isn't cheap and I'll
bet you can kiss your warranty good bye. For the
same amount of money, you could buy a solar system,
which, if you have the space for it, IMO, is a much
better option.

Another issue is that if the pool pump and AC lines are on opposite
sides of the house - it complicates the install.

Solar panels are perfect for our small pool in the desert.

....
Link to video of the episode of TOH
http://www.hotspotenergy.com/pool-heater/