Pilot fish, I think that Bubbles may have replied under the wrong thread. His post would fit with a new thread about diving off cruise ships.
Many divers do take cruises. (Lots of posts about cruises by divers on the "Charter Boats, Cruises & Liveaboards" forum here on Scubaboard) For those with families, a Western Caribbean itinerary with a few good dive ports (Coz, Grand Cayman, Belize) is a great compromise for a family vacation. Many divers who cruise to Coz book their diving on their own instead of just taking the overpriced trip the line offers. Not everyone's situations allow for all trips to be land-based, dive only vacation. A cruise can also be a good way to sample new places and see what you like. I've met several people who first visited Coz on a cruise and now enjoy land-based vacations there. My intro to Coz was on a cruise in the early 90's.
Calica, the port built, owned and operated by Carnival Corp for their lines, <CORRECTION - just used by Carnival, not their port> is about 6km from Playa del Carmen. I've never been to Calica but I have heard that there is a shuttle from the cruise pier to PDC. Depending on the port call times it might still be possible to ferry over to Coz and get in a couple dives. Or, if not, there are some nice dive sites in the PDC and Akumal areas, or try a cenote dive.
Am I crazy about having 7-10 ships a day in Cozumel when I'm there? Not really. But, the business owners, employees and taxi drivers we spoke with when we were there in Dec were sure missing them. The ecomony depends heavily on tourists. With so many hotel rooms out of commission since Oct, and less then half the ships making port calls, it's going to a very tough winter for people who live on the island and need to support their families.