I'm nearly certain that I started a thread on this very topic 7 or 8 years ago.

Since I no longer spend much time in Beijing, my current info set probably isn't very helpful (e.g. 3 of my favorite pools have been paved over since 2008), but I still stand by my general advice:
-enjoy the large public park pools during the first half of the summer only
-target nicer pools in Changping during the second half of the summer
-the Goose and Duck's pool is really quite nice; however I wouldn't bother making the trek unless they are holding some sort of event