I just took that very Cruise in March with Carnival, but did it on the Legend-have you been on a Cruise before?
If you haven't, here's a few notes off of the top of my head-I love Grand Cayman, and there's lots to do-don't miss Stingray City, and the Turtle Farm (Sea Turtles) is great too.
Cozumel, to me, is so over commericalized that it's no fun anymore-they had a Hurricane there a few Years ago-not the same as it was.
Belize is cool, but it is an undeveloped Country (Port)-don't miss the zip line-
Roatan-man, I remember them telling us that it gets to 115 degrees in July there, and the humidity is off the charts which is why we went in March-look for the Rabbits there they are weird-
You will be charged for a Tip everyday, and depending on how much you drink it can get expensive (my Bar Bill was almost $500)-Shore Excursions are not included, as well as any pictures they take-lots of Cuise experiences here:
http://www.cruisecritic.com/reviews/
Hope you got a Balcony-