Business casual for dinner, tanks, t's, shorts, one formal outfit (not needed unless you go to formal night), bathing suit, at least one pair long pants (jeans, sweats) and sweater. It can get chilly on deck at night.
Don't pack your whole wardrobe. There is usually a Laundromat on board or you can send it out for a fee.
No topless sunbathing in the Bahamas, but there is usually an adults only topless/nude deck on board. It's way up high and screened so no one can peak at you. They don't advertise it but it's there, if you're into that sort of thing.
Remember - you don't know these people and will never see them again, so why not leave your inhibitions at the pier and live a little!! That's what cruises are for.
As of Dec 31st, 2006 you'll need your Passport to get back into the U.S. Not needed for Bahamas. There is a form for that and you'll get it when you get your tickets. Fill it out ahead of time and give it to customs when you get off in Bahamas.
Hope you're going to Nassau because Freeport stinks.
Enjoy yourself. You'll have a great time not matter what you do.
Also keep in mind that drinks start at $5.00 and go up. Be careful, it adds up fast!!