I've been on one cruise and that was on Royal Caribbean's Enchanment of the Seas. We took a 7 day eastern Caribbean cruise leaving from Ft Lauderdale stopping in St Thomas, St John, St Maarten & San Juan. I actually did get married on this cruise, which was very well done and managed by RC's staff. I enjoyed the cruise thoroughly (outside of spending my first few days regretting the minor detail of getting married) and would 100% agree to buy your excursions once you get to port. I know when we got to St John's and bought our snorkeling excursion (from the ship) for Trunk Bay, and found out that once we got to port, we could have saved nearly 50% buy purchasing the package on-site. Key point that I would try to suggest would be to book your stateroom in the middle of the ship. Less forward to backward rocking motion, since the stabilizers handle the side to side.
FOOD FOOD FOOD...I could not get enough of the great food. Ask for whatever you want, it doesn't cost any more and it was all fantastic. I probably gained 10 lbs that week alone, and I've only gained 15 lbs since I graduated from HS. You definitely eat well, and if you don't, it won't be because it's not available.