Pack the essentials, one change of clothing and a bathing suit in your carry on, mix and match separates, check your cruise line's dress code, and pack a comfortable pair of walking shoes. Keep all valuables with you. Scan important documents, and send them to yourself on a Web-based email address for easy retrieval.
It can take hours for the cruise line to sort out hundreds of bags and deliver them to guest cabins. It's a good idea to pack a bathing suit and any clothes you plan to wear the first evening in your carry-on -- this way, your vacation can start the minute you're on board the ship. If the airline or cruise line loses your bags, you at least have some clothes to wear. You should, of course, never leave your passport or photo ID, money, credit cards, airline and cruise tickets, medicines or any valuables in checked baggage.
If you're flying to your embarkation port, there likely is a baggage restriction from the airline. Consider packing mix and match separates for a fresh and varied wardrobe on your cruise. Leave some space in your bag for souvenirs. Check your cruise line's dress code, and pack formal attire only if it's mandatory. Caribbean cruises are often informal. There's no point in packing cocktail garb if everyone else is in resort casual.
You may think you're headed to the Caribbean so flip-flops are all you need, but you need comfortable shoes for long days of sightseeing or active excursions such as hiking or biking. It's the Caribbean, so don't forget to pack sunscreen, sunglasses and hats. Essentials usually are available in on-board shops but are often overpriced.