That is where you do the class work and the pool dives at home thru a local dive shop, then, with the letter they will give you, do the 4 open water dives while on the cruise. You get the best of both, plenty of time to do the class work and pool dives, even re-do them if needed, then 4 open water dives in warm clear water. The Voyager class ships have this as one of their certification options.