Bring a jacket or something to layer with as the evenings on deck can be chilly, especially in FL. I about froze around New Years while waiting for the ship to leave Ft Lauderdale. Also, a few of the sea days weve had have been overcast and chilly. However, since youre way up there you will probably consider it to be very warm! Im in SC so my body is used to warmer weather!
We have gone on cruises to the western caribbean the last 2 januarys- 2 yrs ago, it was a little chilly on the ship, you could sunbathe, but we always ended up putting our towels over us. Cozumel was warm, the water was cold, Jamaica warm, Key West warm. Last year it was beautiful the whole time (both times we left Jan. 2 or 3) and we never needed jackets or long sleeves except at night on ship. We also went to Grand Cayman and it was beautiful. Have fun, I love the Western! We are doing Eastern this year.