Cozumel will be quite a bit warmer than here in Tampa. I'm about ready to move south for the winter...except that I'm about as far south as the U.S. goes.
At sea, I would expect it to be pretty chilly the first and last day, especially after the sun goes down. But, as you know, this time of year it can warm up fast. So, I would go with 2 each of shorts, capri's and long pants. I actually prefer capri's even in summer, since they protect the backs of my legs from hot vinyl or splintery wood seats. I would not advise you to only take clothes for just cool or warm, since the odds are high you will experience both.