I'm not shocked to hear about this incident. Just got back from Cozumel 2 weeks ago from 6 days of non-stop diving. We were staying slightly North of the main reef system, so we had to go right by the cruise ship piers on the way to every dive site.
There is a tremendous amount of cruise ship and dive boat traffic in the immediate area, seemingly at all hours of the day and night. It looks like a giant competition to see who can get the most foreign tourist dollars going on all the time!
Our boat night dive and our shore night dive were just beyond the cruise ship piers, and I always had in the back of my mind what potential problems could occur back on the surface (especially since the boat we were on had no lights!). Fortunately, the weather was nice and the seas were calm during our entire stay, and I can only report a wonderful time, great underwater pictures, and an overall great experience.
As others have mentioned, the laws in Mexico are not as strict as in other countries, and the locals will compete fast and furiously for your business. As an experienced diver and instructor, I did not agree with many procedures that were used by Divemasters, boat captains, or dive operators in general, but we all came back safely and in one piece. Just remember that when your mind is in the "vacation mode," some people let their guard down and assume that other people (DMs, etc.) will look out for them. When members of our group ventured off to follow the DM, we always reminded each other to continue to use the buddy system, look out for each other, look out for ourselves, and be responsible for our own safety.
I will return to Cozumel, but next time with a better education and more realistic expectations. It's a great place to dive!
And by the way, most cruise passengers said they wished they had more time to stay in Cozumel to see all the sites. The ships are in port for a few hours, or half day or so, and then back to sea. I tried the cruise thing once before, but for diving, I don't think cruises are the most efficient way to manage my time. In Cozumel there seemed to be cruisers and divers, with the two groups having completely separate agendas. I'll stick with diving, and fly to my destinations.