If you have the time, in Belize best diving from the Cruise ship is in Ambergris Caye.
However, you will need to take a short flight to get to ACaye. After you tender, you will need to take a short flight (35min) (e.g Maya Air) to San Pedro and link up with a good Dive Op (e.g., Amigos Del Mar)...Dive sites are 5 min from the pier. If you do not have the time to fly to San Pedro/AC then you diving will be in/around Belize City.