Remember the ship doesn't use cash, so plan on having them set up your ship account with a credit card. Take some bottled water if you can, the bottled water in the cabin it too expensive.
Keep an eye on your complimentary beach towels (if provided)....you are responsible for them and if they aren't returned at the end of the cruise, you will be charged for them. The professional pictures can be expensive, so only choose the ones you are really going to like. You aren't obligated to pay for them all.
When booking the cruise---always choose the second seating, it will give you more time to get back to the ship from the island and get ready for dinner. Otherwise, if the ship offers open seating, you can go at your convenience.
If you are planning to take advantage of the ship spa, make your appointment first thing when you board the ship, that way you will have the best choice of appointments. (I recommend you set your appointment for a day that you will be at sea---Ships usually sail at night, but when returning, sometimes you have at least one day at sea).
Be sure to pack one change of clothes in your carry-on bag, once you get to the dock, you will be separated from your luggage and there will be no way to determine when it will be placed outside you cabin door before departure.
The Virgin Islands are great for silver, Mexico is great for turquoise , don't buy any fruit or vegetables, You can't bring those on the ship.
Also, usually you are only allowed to bring back 2-4 bottles of duty free liquor, but check with the ship steward before you buy
any liquor.
Finally, you are "expected" to tip all service people at the end of the cruise (matree d, waiter, cabin steward, etc) since they serve you the entire cruise...(assuming you are seated at the same table for dinner every night) Check with your travel agent to confirm whether gratuities are included in your cruise package.
If your interested in a good chartered cruise, check this one out, unfortunately the 2003 7 day Carribean cruise info is not available until August.
http://www.festivalatsea.com/
I know that was a lot of information, have fun, as you can tell I love cruising!!
PM me if you like!