We were in Cozumel and Grand Cayman on the January 26 Magic sailing. It was beautiful. Just around 80 degrees. We went to the beach at Grand Cayman and after 2 hours, I was ready to return to the ship because the sun was so intense. We also went to Chankanaab in Cozumel. After the dolphin swim, I spent most of my time in the shade because it was very warm there, as well.
I think those are pretty standard temps right now but they could fluctuate... just keep an eye on weather.com. I checked every day up until our cruise and it seemed to be right on. As long as it doesn't rain, you should not have a problem.
Jennifer