The weather is ... well ... "variable". We always go in early Dec and we have seen 80+ temperatures that were uncomfortably hot, and have watched Fantasmic when the temperature was just barely above 40. There have been years when we had no rain at all, but one year we had monsoons (el Niño they said) and we had more rain than the entire monthly average.
There are days that are great for swimming, days when it's OK during the afternoon, and other days when the lifeguards at SAB are freezing their butts off with NO ONE in the pools.
But in general the weather is moderate, and it's a great time of year to go. Late Nov - Early Dec is great because Christmas decorations are up, and there are very few rides closed because they want them all open over the holidays. There are a lot more closures in Jan. Typical only one of the water parks is open.