Last summer my son (13) and I got certified so we could dive with my husband on a cruise to Cozumel and Belize over Thanksgiving. We had maybe a half-dozen dives (not counting the cert dives) in the quarries around Houston before we went for the cruise. That was barely enough to get us comfortable on the dives. And salt water is WAY different on buoyancy!
I did LOTS of research and discovered that it is MUCH better to dive through the cruise line than an independent operator. First, and not to be scoffed at, is the guarantee that the ship will not leave port without you. If you book through the ship, they will wait for you, no matter what. If you go with an independent, you have to find your own way to the operation, and worry the whole time you are diving that you will be late. In Cozumel, it was only about $10-$15 more per person, and that included the land transportation. Second, cruise ships are undependable about when they actually get into port and when you can get off. You can never get off before 9 am (and usually around 10). By that time, normal dive operators have been under water for hours. You will find that scheduling is a big issue with diving from a cruise ship, unless you do it through the ship.
As for dive gear on a ship: as you know if you have cruised before, the bathrooms in the cabins on the ship are beyond tiny. It was interesting to see our cabin after a dive: 3 wetsuits, bc's, fins, and regs draped all over everything in that cabin. It was a real pain, especially rinsing the salt water off everything in that postage stamp of a shower, but I didn't trust rental equipment in Mexico, and we are an especially tall family so we don't fit normal wet suits, or even fins for my son's size 15's! However, another couple on that went on every dive that we did rented everything, and had no problem. They got back to the dive shop, dropped everything in the tanks, and walked off. If you are a normal size, then this option is definitely more convenient. But at least take your own mask. It takes up no room. Next item to bring would be your own reg.
I had lots of questions about the ship excursions beforehand, and called the cruise ship main excursion office. They were extremely helpful and got back to me in a day or two with all the answers. They won't give you the operator's name so you can contact them directly. They think you will go around them and book directly. But at least in Mexico, it was worth going through the ship. I can't imagine the schedule being any better anywhere else.
Hope this helps.
