I prefer 77-78. I like nothing more than when it is about 85 and sunny outside and the pool is in the high 70's. Unfortunately, that only really happens in April and early October.
In mid July when the water is consistently in the 90's, I barely get in. It's about 100 degrees outside in the afternoon and humid, and the pool doesn't even cool you off. Kids still swim though. Thankfully, it's starting to cool off again.