My sister-in-law and her family came to visit us when Nora was a baby. It was April and it was fairly warm, like in the upper 80s and 90s. We took them to the pool. Now, I never go to the pool in the "off season" mostly because during the summer months, we spend so much time, so, so much time at the pool, I get really sick of it and have to take a 7-month break. However, what we discovered on that sunny day in April was that for Southern Arizona, it was too cold to swim.
It wasn't that we were being wimps either. Well, maybe we were. But I really think the low humidity makes the water evaporate off your skin faster, sucking heat off your body and if it's not 105 degrees, it gets really cold.