Sea conditions don't follow a calendar. Seas in December are no more likely to be rough than seas in June. There is always someone who, because they experienced rough seas on one cruise X years ago, runs around telling everyone it is always like that if you cruise in (insert month). Don't believe it.
Could you encounter choppy waters in December? Sure. But then so could I next week when I'm on the Carnival Elation. It isn't something to waste your time worrying about.