Stoney3K's point about the rates being cheap is a good one. It's the primary reason ILDA has decided to hold their conference on the cruise ship again this year. It would actually cost more to hold it at the DisneyWorld resort (which was the other choice), and if they did that you wouldn't get free food or the fun of experiencing the cruise itself (not to mention the cool places you'll visit).
Cruises in general offer considerable value. Even the higher-end cruise lines are still quite affordable. I went on a 7 day cruise with Royal Caribbean on one of their largest ships back in 2005, and even so it only cost me around $1300 per person. (Again, that was for an interior cabin with no window. Exterior cabins always cost more, and balcony cabins more still.) And for that, they treated us like absolute royalty. Even now, 5 years later, I still remember the names of our waiters and cabin stewards. It was like they became part of the family for that week.
Granted, Carnival Cruise lines doesn't offer that same level of service, but it's also 4X cheaper! And there will still be great food and plenty of on-ship activities to keep you busy. (Not to mention the fun you can have in port!)
If you're thinking about adding some "after-cruise" fun to your trip, remember that Orlando is only 4 hours from Miami by car. So you could head to Orlando (either before or after the cruise) and go visit Seaworld, Universal Studios, Disneyworld, or one of the other attractions. Also, the Kennedy Space Center is roughly the same distance away by car, so if you'd like to visit the space coast instead, that's another option.
Adam
PS: Sheesh! I really should stop this... I sound like a damn travel agent!