There is no way to predict it. You may be able to swim and you may not. We have been in December the last 4 years. In 2008, they had a heat wave with record highs the week we were there (88 one day), and we were sweating all week. Definitely warm enough for the pools any time that week. In our 3 trips since then (all 3 of those were the 1st week in December), we started off the week in shorts and the first day or two it was warm enough to swim, but we ended up those weeks in sweaters and jackets and there was no way we would get in the pool, heated or not. It is just a matter of luck. There is a wide range of weather that happens in December at WDW and the weather can change fast. You can't trust the forecast even if you look at it an hour before you leave for Florida (we found that out the hard way). You have to go packed to be prepared for any type of weather.