From what I have read here on the board over the last couple of years, the weather generally isn't a factor in the western Caribbean at that time of year. However, with any storm at any time, you can have some rockin' and rollin' seas. We were lucky (?) enough to hit a big cold spell coming down off the US into the gulf of Mexico the first week of January this year on our Carnival Pride inaugural cruise. Do you remember a big cold front that hit Atlanta and (I understand) closed the airport because of ice on the runway? Well, that cold front moved on down into the Caribbean and rocked our ship pretty good starting about 11:00 p.m. Wednesday night, Jan 2, as we were getting ready to leave Cozumel. We rocked and rolled all the next day (a sea day) and then couldn't tender into Key West on Friday because of the high seas, so we had another sea day before we returned to Miami the next day. So, to make a long story short, storms can happen any time of the year, but my understanding is that it isn't usual for that area in December.
We sailed the western last December 9th and it was probably one of the smoother sailings I have been on, with the exception of entering the mouth of the Mississippi but I understand that is rough regards.
The water was a little chilly when first entering but nothing that was untolerable.
We like to dive and the water is just a little chilly that time of year - not that I wouldn't go mind you but they can get cold down there to. One dive group went to Cozumel during that time of year and Cozumel had a freak cold spell and it was in the 50s. No diving for that group! Debbie