My husband and I cruised the western carribean om December (the week before Christmas) a few years ago, sailing out of the Port of Houston (not a Carnival cruise that time). The weather was quite cold the night/day we left, but when we awoke the first morning, we were far enough south that it was balmy and tropical....and it stayed that way the entire cruise, until we returned. I'm trying to remember if that last night before arriving back in Houston, if it was coolish....it may have been, but I don't really remember it. It was definitely cold when we woke up cruising along the coast of Texas!
Hope you have a wonderful trip! What are your ports of call?