I have been diving with Martin for a couple of years now and really like the operation, very good DM's & captains; small, fast, reliable boats with 6 divers per DM max.
He also has some the best prices I've found on the island, cash only, also if you can get your hands on some used prescrition glasses he is collecting them for his charity this year and I belive with 2 pair of glasses you get some kind of extra discount, check his website to be sure.
When is you ship scheduled to arrive in Coz? The reason I ask is Martin's boats leave at 7:30a.m. for morning dives so if you are not coming in on the day of your dives check into afternoon dives as you may not be able to disembark from the cruise ship in time to make the a.m. dive.
Like Parrothead said, the DWM dive shop is right at the foot of the International Pier so if your boat docks there or at the Mayan Pier you are within walking distance.
If you dock downtown Blue Angel would be closer, but not as short of a walk. Although I think both will probably pick you up at a boat pier in the downtown area for the dive if you are coming in downtown.