Here's the deal in February, coming from a guy who lives in South Florida. The stop at Castaway Cay is going to enjoy the same weather and temps that we enjoy in South Florida, as the island is just a short ways off shore. I have seen hard frost in South Florida in February, but not EVERY February. Plan on daytime temps between 50 and 75 degrees, with colder nights in the WORST of weather. Normally you need a jacket in the morning, but it warms by mid-day, kind of like Michigan in MAY. Once you get down to US and British Virgin Island, you are a LONG ways south, and like they said above, it'll be warm and humid with a chance of rain most days.
WE live in south Florida and WE are cruising on Fantasy in February. I find the Virgin Islands too darn hot other than in the winter.
Hope this is a help.
Tom in Okeechobee, FL