There are several Excursion Packages offered by Carnival. You do not mention your level of experience at snorkling however. They do have some wonderful areas in Nassau to snorkel. One of my favorites is Blue Lagoon (which a lot of people refer to as Gilligan's Island. Very nice beaches and of course 2 nice lagoons.
Have not been to Blue Lagoon for many years, but I liked it back then. The lagoon is nice..not many fish, but you can walk over to the wave side and are more fish near the rocks.
Take care of that ship! She is headed to Cozumel next for my first Palancar Reef snorkel
There is a beach near the ship. The problem is that it is not in walking distance so you will have to take a taxi. There is also Paradise Island. I think if you check there will be excursions for it there and Atlantis. Have fun.