Two yrs ago I did this in St. Thomas. The Coki Beach Dive shop offers a beginners dive. It was totally awesome. You can check them out on their web site <www.cokidive.com>. There is a section for cruise passengers.
I went on my own. Took a cab to Coki Beach. Got there in time to join the class. The class is about 45 min. with a quiz at the end. Then you are fitted with the equipment. I had my own mask and fins. Once in the water there are some more instructions and practicing. Then it is off to the reef. We were down about 25 feet for 30 minutes. Absolutely marvelous. I would love to do it again.
The cost was $50.00 for the dive, $15.00 for the cab and $17.00 for the shirt I just had to buy. The cruise excursion was $99.00 so I saved a bit. Plus there were only 5 people in the dive with two guides.
It was worth every penny. I will be in St Thomas in Oct and am thinking of doing it again.
Have a great cruise.