I took a NCL cruise a few years ago in Hawaii (island hopping) and did 3, 2 tank dive excursions, booked through the ship. They just contracted through a LDS, no ship board DM. All in all it worked out well but had very wide differences in the LDS setup. 1 was very well organized, setup and run, great shop and nice well stocked boats with clearly knowledgable DM's who could point out and identify everything imaginable under water. Great dives. One of the others was literally run out of a lean to shack on the beach and made me extremely happy to have brought all my own gear. Tiny, tiny boat with 2 guys that talked like used car salesman. Against my better judgment, and with my wife giving me very descriptive looks we went ahead with the dives and actually had a great time and included my first encounter with a large white tip reefie. I guess long story short, most cruises are going to sub out through a LDS so play it by ear, use your best judgement, but know that even despite the size and setup of an operation, if you're in blue water on a vacation you will likely enjoy the diving anyway! Good luck and have fun.