Hi Catlover1213, and welcome to cruise-chat.com!
You might find answers to some of your questions on our Caribbean chat board, located here
. Sorry that I can't help with Roatan or Belize...haven't been there.
If you have your own masks and snorkels, I'd bring them. If you go on an excursion through NCL, they may still make you rent a vest through them, but at least you'll still have your own gear that fits correctly (and is more sanitary).
We've found that a ziploc works well as a waterproof container. You won't be bringing much ashore -- ship card, one credit card, photo ID (possibly), copy of first page of passport, a little cash, port agent info -- all of which can fit quite nicely into a ziploc. My husband's swim trunks have a pocket that zips, and we put the ziploc right in there.
If you are going to eat off-ship, get some recommendations from the crew. We've found them to be quite helpful.