We rarely purcahse excursions from the ship. We like to do our own thing while in port and don't want to have to adhere to a schedule (other than when the ship actually leaves). I found Frommer's Caribbean Cruises and Ports of Call to be a good resource. The book has a separate section for each Caribbean port, with sub-sections for beaches, shopping, tours and transportation. You can book your own tour or excursion, or just use it as a guide to plan your own thing.
We used it to figure out what we want to do, and still have an old copy somewhere, in case we find ourselves heading to a new port. I usually pack it for cruises, too. If we end up getting re-routed (as we have in the past), we've got a resource to help us decide what to do while at the unscheduled port.