I've been on two cruises with Carnival, Eastern on the Glory, Western on the Dream. Dove with the Carnival tour groups on both.
My reason with booking with the cruise line is this....they guarantee that the boat will not leave port without you if you book with them. If you choose a private dive operator, and have a 10 times better dive experience, and save maybe $25, and go to maybe a little better dive location, and miss your boat coming back, think of how that will screw up your trip and how much that will cost.
All of my Carnival dive trips were great. I say book with them, go diving, come back to the ship, and go to the casino that evening and play craps. I can tell you how to win if you don't know how.
Have a great cruise and dive vacation.
Darrell