Hi,
Just went on a cruise on Carnival (Jamaica/Grand Caymen/Cozumel) from Nov 27, through Dec 4. No diving was offered by the cruise for Jamaica, but I bought the diving excursions for Grand Caymen and Cozumel. I was comfortable without a wetsuit in both places, but most everyone on the boat wore a wetsuit (not sure of the thickness, as you can imagine how awkward that would be for me asking). I have always dove Cozumel (August, November, and November/December) without a wetsuit, but I have not dove there in January to know if the water gets colder.
Although everyone, including me, will tell you that the cruise ship excursions are pricey, the extra cost of booking the excursions through the cruise ship, versus going on your own, is just cheap insurance to get you back on the ship before it leaves you behind and you find yourself paying a high priced airfare to get you to the next port to get back on the ship and back to your luggage.
My oppinion of Grand Caymen was extreme safety, with strict adherance to diving well within the limits of the dive tables with a factor of safety of adding a 10 ft minimum depth allowance for planning the "no decompression" bottom time, even at divesites that have a shallower bottom.
Grand Caymen cruise ship pricing was competitive compared to going on your own, but still slightly more expensive than going on your own. Cozumel cruise ship pricing was more expensive by a factor of almost 2 compared to what you might pay if you went on your own without any time limits. However, the time you get into any of these ports of call do not correspond with the regularly scheduled dive departure times that the dive shops run their business on (morning or afternoon), which means you have a high chance of missing the boat when it sails into the sunset.
Hope this helps.