Hi hawkeye, and welcome to Cruise-Chat.com!
You can find a good deal of information on your ports and shore excursions on the Cruise-Chat.com Caribbean Port Chat board, located here
As for your diving question, I can't help in tht regard, but NCL has been running its Dive In program for many years. Hopefully someone who has used them will respond to your question.
I've been to Great Stirrup Cay a number of times, but have not taken a shore excursion there. (I sunned, swam, ate...) Was there anything on the NCL site that interested you?