Gail - sailing the Eastern in February you may need a pair of lightweight pants for outdoors in the evening for the first night or two. A lightweight sweater or jacket is also a good idea. Once you hit the warm waters of the Caribbean you should be able to wear shorts on deck at night
The best thing I can tell you is to be prepared for anything. For the Carib in August, you should be fine in shorts, but if you tend to get cold, you might want to wear a little more on your legs and something around your shoulders.
Cruising in February is a little more interesting. You might have to wear your winter clothes on the day of departure, and by morning, be in shorts.