We're on the August 30th cruise as well - it is myself, Tenley (23) and my sister Tiffany (18...19 on September 7). We have the late dinner seating. I am also a DVC member! There actually are quite a few people from the boards going as well...
To answer your questions - yes, you can bring alcohol on board, but if you choose to bring wine to dinner, you have to pay a pretty hefty corking fee (I want to say it is around $18) plus gratuity.
As for snorkling, I know there are several excursions in Grand Cayman that take you snorkling and to stingray city. We are going on the 3-hour afternoon tour with Disney that takes you just to Stingray City.
Hope to see you on board!