I went on one of the cruise sponsered dives in Nassau a couple of years ago and it was not the best experience. The dive site consisted of taking a boat out of the inlet and just around the corner. We weren't more than a couple hundred yards from shore and the cruise ship never left our site.
The worst part about the experience was that the air made me sick. When I was down on the dive I noticed a strange "taste" to the air and then I realized that they were probably using old filters at their air station. I felt pretty sick the rest of the day.
Personally, I will never book another dive excursion from a cruise ship, they are definately overpriced for what you get. Try going to your certification agencies website and research some of the dive centers around there and read reviews, and book direct. It will likely save you money and you'll have a much better time.
Good luck and happy diving. =)