Welcome to ScubaBoard! If you haven't yet arranged for diving while on your cruise. . . You may want to search the board for messages from divers experience where your cruise ship will stop. At the bottom of each page there is a button called "forum jump". Go to forums for Mexico/ Cozumel or Greater Carribean, and then use the search feature for Cruise ship diving on the island you'll be out.
Recent messages in the Cozumel forum have listed several dive shops that will match the ships schedule and offer cheaper prices and more individual attention. This may not be of interest to you, but it is available.
You don't mention your diving experience in your profile. Let us know if you are starting out or have over 1,000 dives like me. . . It's to let us know how detailed our answers should be,whether to recommend equipment you need, etc. This board has lots of beginners and lots of experience.