Luggage and bags:
If coming by boat (which I recommend if you have an appropriate boat) you don't have to pack. Except for provisioning the boat of course and filling up with water and fuel before you leave. If you come by air, pack light. If you come on a dive boat, I'm sure they'll tell you what to bring.
Clothing/Shoes/Weather Gear: If you are diving or snorkeling, pack bathing suits, dive skins or wet suits. Otherwise, almost any type reasonable body covering will do. Sensible folks wear a hat during the day. You may need shoes.
Toiletries and Medical Supplies: Bring what you need - it may be possible to buy it there, but it's not an ideal place to shop for stuff like lipstick or aftershave since you can't even get much in the way of postcards and you can't always get ice cream.
Photo Equipment: Dive or snorkel cameras if you are going to do that. For a film camera, bring double as much film as you think you will need and fresh camera batteries.
Camping/Beach/Outdoor Gear: If you are diving, bring your stuff, although some of it can be rented. If you are fishing, bring your stuff.
Miscellaneous: If you have access to a VHF radio, it is like listening on a island party line. Don't bring phone cards from home and expect to use them (unless home is the Bahamas). I've never been able to find an internet connection except at the dive shop.
Toiletries and Medical Supplies:
Bug Repellent is a must if you care to watch the sun set or rise. Benedryl Gel is great for bug bites and even jelly fish stings. The best cure for jelly fish stings or scratches from fire coral is a salve of meat tenderizor and water.
Photo Equipment: Disposable underwater camera to capture the multitude of sealife, including friendly dolphins. Don't miss the chance to get a shot of the Chalks seaplane landing and taking off!!
Camping/Beach/Outdoor Gear: Large beach towels a must.