We're cruising in January of '08, as you can see by my signature. My first 2 cruises, were 3 and 4-day cruises, in Mid-December and early January. We had good weather both times, might be a little cool for some, it probably varied between 70's to mid-80's.
Are you thinking of doing a 7-day cruise? If so, eastern or western? I would probably recommend eastern if you want some warmer weather, as even though I haven't been there yet, from what I've read, it's warmer on the eastern because you go further south than you do on the western. I've never been impressed by the weather in Florida the times we've been in January, so maybe that's why the previous poster had some bad weather there...who knows. it IS winter.
As for castaway cay, we snorkeled a bit, and I got a little cold after awhile, but for the most part, it was fine. We couldn't dock at Castaway our first cruise, in December, which was a drag, but I'm pretty sure that's quite rare. We also snorkeled in Nassau once and that was fine too. I AM from Minnesota, so that could have something to do with it, but it isn't exactly the frozen Siberian tundra that some in warmer climates think it to be
I guess it's up to you. No cruising time will probably be perfect for everyone. It's not off-peak season for nothing

whatever you do, I'm sure you'll have a good time.