Unfortunately, cruising can bring with it the unpleasant side-effect of seasickness. While many people won't suffer from this malady, those who have can attest to the fact that packing certain items may well have saved their vacations. Bring an anti-motion sickness remedy like Dramamine or Bonine. Yes, these medicines will be available on the ship, but they'll cost substantially more than they will at your local drugstore. Also pack a supply of other over-the-counter medicines like pain relievers, acid reducers and anti-diarrhea medications. The water on the ship will be filtered, but if you drink water or eat some foods from other countries, you may be at risk for developing a raging case of diarrhea.
Most Caribbean countries are known for their abundant outdoor markets. Nassau's infamous Straw Market, for example, is a shopper's paradise, with everything under the sun made of straw. Baskets, hats, handbags and even skirts are for sale at this outdoor mini-mall. In addition, an abundance of native crafts and typical souvenir items are available at significantly lower prices than what you might pay at home. The natives, however, don't always have paper or plastic bags to carry away their goods. Come armed and ready with lightweight canvas or nylon shopping bags. Fold up a few and stash them in your suitcase. They will be helpful when disembarking the ship at the end of your cruise. They can also hold items like wet bathing suits, keeping them away from the rest of the items in your luggage.
If you think you'll only need one bathing suit on your Caribbean cruise, think again. The same goes for that second pair of flip flops and those extra t-shirts you thought of packing. Whatever you're certain you'll need, count on packing extras. Even though the climate is controlled inside the cruise ship, it won't be once you're outside. You'll be waiting for your bathing suit to dry while wishing you had another to wear to the pool if you don't pack extras. Caribbean climates are hot and humid. You'll likely change clothes more often after trips to many of the islands. You can buy extra shorts, bathing suits and t-shirts in the cruise ship boutiques, but you'll pay dearly for them. Save yourself both the frustration and the expense by packing extras from home.
Don't forget that most cruise ships host meals and events that require formal attire. Check with your cruise line so you don't get caught without proper evening wear for such events.