We just went on a 7-night Eastern Jan. 31. It was really chilly (we wore sweaters) while we were in Port Canaveral and on our way down to St. Croix. We had two sea days before our first stop, and it was pretty chilly both of them, and rained some. We were able to swim one of the days, but I'm a baby when it comes to cold weather, and I just didn't enjoy it as much.
It was gorgeous in St. Croix and St. Thomas. We went swimming in St. Croix, and it was warm and very comfortable. It rained on and off the next day in St. Thomas, but it was still warm. Then when we got to
Castaway Cay, it was too cold for me to get in the water. My daughter went in briefly, but soon after was wearing a t-shirt to stay warm.
All that being said, it was really a wonderful trip. Sure, the weather wasn't great, but we had a blast anyway. And we live in Florida, so it's not as if we really need to swim a ton. And I remember reading posts from people who cruised the week after us and they said it was gorgeous - it was really nice and warm at home that next week too.
I guess it's close to impossible to guess what the weather will be like. We're going again in Sept, and I suppose we risk a hurricane or something. But we'll deal with it if it happens. I guess it's tough to guess when the weather will be nicest - no rain, warm enough, etc. But I would think your chances are pretty good in February in general.