Our last trip on Mariner was wonderful-you'll love the ship-she's everything you'd expect. When we went on the Eastern cruise, the ship then stopped in Nassau, St. Thomas and St. Maartin. I understand Coco is the same as Labadee, Haiti, which is a Royal Caribbean port-I assume it's got beaches, play areas for the children, and the usual over pricede trinkets and junk usually available at any other port. The usual port lunch BBQ provided by RCCL is very nice. St. Maartin has some nice areas and things to do also. We went snorkling at one of the beaches on the French side of the island (if you're really a little prudish, you may want to avoid the beachs on the French controlled side of the island-they are optional topless).
As for Mariner, she is the newest of the Voyager class ships-try for a balcony room-either side of the ship-it's well worth the extra money. A standard D2 cabin is more than enough room for 2 people to be very comfortable.
Have a great time!!!!!