My experince on a cruise ship dive was November of last year. Royal Carribean Cruises. It was a single tank dive at their "private island" stop over. A little disorganized, as at the set up area, we basically had a feeding frenzy over which tanks were available. Me and my instant dive buddy ended up with the smallest tanks while the smaller divers got the biggest. Needless to say, we were the first out of air! But surprisingly, just by about 3 minutes which made me feel a little better.
They towed us out to the start of the dive by hanging on to a sled behind a jet ski. Now that was fun!! The viz was low. 10 to 15 feet because of a recent storm, which wasn't bad to me as I usually dive quarries and lakes anyway. Depth maxed out at about 40 ft, and we did dive through some "valleys" and saw a lobster and a few neat corals but as an above poster stated, they were pretty banged up reefs and hardly any fish to see. But remember, the viz was low.
Add to this, it was my first salt water dive so I was stoked anyway. I would have dove in 1 ft viz just to say I had a salt water dive!! LOL.
They did forewarn us that the viz was bad and we would get a refund if we wanted. A couple of experienced divers did back out. I do believe though that different cruise lines offer different levels of diving excursions and a little homework will go a long way.
If you're looking for that great Caribbean dive, you may be better off arranging a dive through a local dive shop where the ship docks up for that day.
Having said all this.....it was still a highlight dive for a beginner diver and I'll always remember it.