In our experience (limited as it may be), for Georgetown, a MUST do and see at least once is Stingray City. Also, in Cozumel, there is a rather pricy Royal Dolphin Swim and Meet at Canakanab Park (I'm sure that it NOT spelled correctly). I've seen raves, and some not so raves, about it. We're doing something like that this year in September.
Have a GREAT CRUISE! We can all agree, you'll be hooked!!