Look at our Dress FAQ
which also has a link in it for RCI's policy. During the day it is casual everywhere, every day. It is really weather dependent. For me it is typically shorts and t-shirt in the Caribbean - no matter what month it is.
The Caribbean can be cool, warm, rainy, sunny, windy, calm, and overcast. All in the same day. Nobody can say for sure what it will be during your cruise. Want averages? For February in the central Caribbean the temperature will run from 70-80 degrees.