I'm back from the cruise, the first night was a little chilly (but like some of the previous people who posted I always wear shorts unless going to the MDR so I guess cold/warm is subjective). Blue jeans and a light windbreaker seemed to be the norm on the ship day 1. I was one of the very few wearing shorts . The first sea day morning we were about even with Miami in terms of latitude and the temp was just above 70 F.
I'm sure you are probably aware, but there is a solarium area on the ship where the glass roof can be closed and opened. It was very comfortable the entire cruise (if not hot while in Costa Maya and Cozumel). I only saw the roof retracted while we were in port in Tampa. I swam every day in the solarium pool and the first sea day the water was a bit cold in the morning, but the sunshine really warmed the water up quickly! Pool temp was nice every day (to me).
The remainder of the cruise was warm. Costa Maya, Cozumel and the final sea day there was no need for a jacket. The crew of the ship went overboard to make sure everyone was having a great time.
We had an awesome time. I hope you enjoy your cruise!!