I've been there in December twice. It was about 26 all week for the first time and about 23-25C the second time. Went scuba diving that second time and the water temperature was about 19-22C(down where I did it).
Personally, it's better to plan your wedding in Cuba in December than in September-October. That's when the hurrican season is in full effect and you'd be gambling big time.
Cheers,
Dmitri