While Captain Marvin's is a well established snorkel tour operator, I do not belive they conduct scuba diving trips.
Shore diving is available at Sunset House, Eden Rock, Don Foster's, and DiveTech.
Many small dive operators can adapt schedules to fit cruise ships guests. Check with Ambassador Divers, Neptunes Divers, Wall to Wall, and others.
Finally, do a little research on dive specific websites such as Scubaboard. Be sure to use the search function as these questions have been asked hundreds of times!