Princess Cruises offer a fantstic wedding package, and you can either get married onboard or ashore. They have Florida based wedding coordinators who are very helpful at arranging everything for you. If you do decide to get married onboard you friends and family can watch back home as they even set up a live web cam.
A company called theweddingexperience.com will handle all the details. Get in touch with them.
One caveat. Go to the Caribbean between April & November & you run the very real risk of the ship having to miss out that island due to bad weather. Missing out an island has happened to us a couple of times, no real problem if you are on holiday, you get an extra day at sea, or a 'replacement' island, but if you have organised a specific island, well...............