I may get flamed for this....
My wife and I went to the Republic of Panama for vacation in January. We returned to the Tampa Bay area on Super Bowl Sunday (Feb 2). That week the weather was COLD. Lows in the 20's. The pool filter pump was running every night all night for freeze protection. On Thursday morning, when I checked the water temp, it was 64F.
However in this area we have a saying: "If you don't like the weather, just wait!". The following week (last week), it was much warmer with highs in the mid to upper 80's. The pool started to warm up due to the solar heater and the clear warm days. By Wednesday the water temp was up to the upper 70's. I told my wife that if the weather held, we could possibility go swimming over the weekend.
On Friday my elder step son checked the water temp and found that it was 86F. He immediately came over and he and his son jumped in. When I got home, I was also hot and so also jumped in. We were then joined by my other step son. First pool party of 2009!
This week the weather has changed to the cooler, with air temps in the 70's. A bit too cool to go swimming.