I usually cruise the Caribbean once every three years or so, last time was in October on our second trip aboard the Disney Magic (awesome cruise boat!). I never use the dives set up by the cruise line(s). Very pricey and in my experience you're hurded like cattle no matter what your qualifications. El Orans listed some very good operators (I used Neptune's Divers in Cayman). If you contact the dive shop(s) ahead of time they will give you directions to their locations and a few will even provide pick up. It is easier to go with what is offered by the line, but I've always had better and less expensive diving when I made my own arrangments.