Once you get away from the mainland, it's nice and warm. Not too hot, but comfortable. Fewer daylight hours, though,as it's winter time. I went to the Western Carib. in mid-Dec one year, and over Christmas a couple of years later.
What ship, and where are you leaving from? I sailed out of New Orleans and port Canaveral. Both times it was pretty chilly leaving port. Out of New Orleans (Dec. 15, 2001), the first day at sea was rainy and cold. By the second day at sea, it was beautiful. Great weather in Cozumel, GC, and Jamaica. Out of Port Canaveral (over Christmas 2002), it was literally freezing when we left. Byn the time we got to Cozumel, it was beautiful. Also great weather in GC, Jamaica, and Half Moon Cay, Bahamas.