It's a personal preference. Usually for heavy swimming, pools are in the 78ºF-82ºF range (the Red Cross recommends 78ºF for competitive swimming). Anything warmer doesn't feel comfortable with a heavy workout. For just plain recreational swimming, 82ºF-86ºF works for many people with personal preferences varying as to what feels too warm vs. too cold (the Arthritis Foundation recommends 83ºF-88ºF). For therapy pools or ones where people want it feeling warm without any chill, but don't plan on doing long swimming or heavy exercise and may need to stay in for an hour or longer, 86ºF-90ºF works (especially 86ºF-88ºF), though this feels like a lukewarm to warm bath. Anything above 90ºF starts feeling more like a hot tub than a pool though truly "hot" tubs are up to 104ºF. People's preferences also vary depending on the air temperature.
Personally, my favorite temperature is 88ºF, but I'm skinny and just lounge around in the water and like entering the pool without any chill feeling. My wife does therapy exercises (mostly stretching and walking with only some swimming) so this temperature works for her as well, but most people would find this temperature too warm, especially if they swim. It doesn't cool one off until one gets out of the pool and has the water evaporate from one's skin.