diveborg:
I am of a different opinion. I think you should make your own arrangements.
Another thing to consider is a combo dive/snorkel trip. If you are a mixed group of divers/non-divers this would enable you to stay together.
Blue Angel offers a combo snorkel/dive trip that departs later in the morning that would work with a cruisers later arrival time.
In Grand Cayman
www.neptunesdivers.com is a good choice, even for newbies. They do guided dives if you want and they have a max of 8 divers on their boat.
I would have said to book your own thin too, but that was before I tried diving while on a cruise. My friend had booked the dives through the ship for us, and I razzed him a bit, saying that I would be taking care of this at the remaining stops. We had been re-routed to Nassau instead of Hati, so it was a bonus dive day anyway.
Dive was fine, not a private charter, but not a cattle boat either. On diver went solo and too deep. He had drifted well away from the group and was about 30 seconds from being left behind if not for a dilligent roll-call/head count by one of the crew. The captain cruised over to get him, the crew got him aboard and on O2, while the monitored him for any problems. He was fine (we saw him on the cruise many times after, and he had been checked out by the ships doctor), but all of that took time and we had already spent too much time at the site.
Needless to say, we got back to the dock late, by the time everyone had their gear off and loaded on the bus, we were overdue for our departure. Our driver did his best Dale Earnhardt impression and got us back to the ship. No customs check, we were waived through the gates and up the ramp to the ship. I didn't make it to my cabin before the shp was sailing out of port.
At least from what I was told, had we not booked through the ship, we would have been left to fend for ourselves.
I will never try to mix a cruise with a dive trip again, and this is just one of the reasons. I hope my story was illustrative. What some passengers did was tag along with us to the dive op, then negotiated a better rate for the dive and had the best of both worlds since we all had to get back to the ship, but they cut out the middleman.