I went for spring break once and nearly all the pools were open but not many people got off the lounge chairs. It was plenty warm to lay around but the water was chilly.
I was in Vegas in July when it was 108 outside and windy. We went to the Bellagio pool and got into one that no one else was in. It was freezing: it had a fountain in the middle of the pool that was circulating the water and was so cold it was unbearable.
Walked by the "adult" pool at the Encore three weeks ago and saw a few people swimming. While having breakfast at the Terrace Point Cafe in Wynn's, saw a woman swimming in that pool.