My kids will swim in 75F water easily. Me and the wife prefer 85F. Low RH in southern AZ means very rapid evaporative cooling when you get out of the water. My middle boy (the string bean) will go Smurf blue if I don't towel him right away.
84 if it's hot and sunny, and even then the littlest will start teeth chattering after a bit. 84 is about what I'll heat it to and what it can then maintain without additional heat by leaving the solar cover on 24/7 with air temperature at a high of 90+ and a low of 60+. Once the low overnight goes to 70, I pull the cover and it doesn't go back on until monsoon season.
87+ is my "normal" swim temp through the season and can be maintained with no cover at all as long as the overnight temps stay above 70.
In the desert, I've found that I lose more heat to evaporation than I gain from having direct sun exposure on the water - hence the blue solar cover is usually on 24/7 or stored in the garage. When it's time to replace old blue, I'll try a clear one and see what differences it makes in heating/maintaining heat.
A lot higher than when I was a kid that's for sure. Low 80's is tolerable but 85 to 87 is ideal. For the spa, I really like it right around 100 even, but do like excursions up to 104 or so for short spells.
But it's a dry heat! That's why it is sucking up all the water and cooling your pool. I'm thinking of eventually doing solar panels to counteract that but is it worth it? IDK.
My wife and I like it to be 85+ but the kids will get in anything above 72. It was windy and on the cold side here in Houston the last few days (55 is cold for us) so that the pool felt warm at 82 the last couple of evenings. So I guess when it's cooler outside the water may feel warmer versus when it's 105 outside and 82 feels like freezing.