My family and I have been there twice in December, both times the first full week of the month.
The weather can be hit and miss at that time. The first time we went, during the land portion, it was very cool days in the morning and and late afternoon. There was just a short time in the afternoon where you could take off your jacket.
While on the ship it was cool winds and some rain for the first couple days. We were lucky; on the full day at sea it cleared up and was beautiful.
On the second cruise it was beautiful weather the whole trip. The water was cool at CC but we still got in and snorkled. We did get use to it shortly after getting in; but it was a bit of a shock at first.
Hope you have a great trip!