If you plan on doing any snorkeling on one of your excursions, and plan on taking any underwater pictures, buy a disposable underwater camera at a discount store such as Walmart or K-Mart. It will be much cheaper than if you wait and purchase one in the ship's gift shop. Also (very important!), don't forget your sunblock. A sunburn is not the kind of souvenir you want to bring home.
Hope you have a great cruise!