Carnival will let you book on the boat. It may or may not cost more than something you arrange yourself, but you can be confident they have checked out the operator, they will take care of transportation, equipment, etc., and there will be minimum hassle. That said, they may not offer the kind of diving you want (it may be geared more to beginners or large groups), they may not have it at all locations and you don't get flexibility (in terms of dive spots, etc.).