After reading what RCI has on their website, I have to disagree with browezilla. RCI DOES offer PADI Openwater Certifications, the full deal.
You CAN complete the open water dive certification on their Voyager class ships.
Here is the link to the page that tells about it:
In my opinion, the referal is the best way to go. You don't spend time on vacation doing the book work or the pool dives, just the 4 openwater dives. You also get more time to do the class work and if you need more pool time, it is much easier to get it locally than on the cruise where you have a definite deadline to complete the pool work before the openwater dives.
Check with your local dive store for more information about the referal