Hey Tammy,
You can snorkel off any beach (Cable beach in Nassau is nice. About 10 - 15 min and about $7.00 in a cab) but you're not going to have the same experience as going out on a boat to Blue Lagoon Island or someplace neat like that. There are locals that will take you out but I'd stay away from that unless they can show you things like life vests, radios, fire extinguishers and, yes, a ladder or swim step. Susan was in Veracruz a few years ago and went out with a local boat that had none of those ammenities. When they were done snorkeling the boat crew just "hauld them in like flounders"!
Susan and I are also experienced snorkelers with our own equipment but always pay for the shore excursion to go out to the reefs. IMHO you can snorkel off the beach in Miami. You're going all the way to Nassau so be safe, do the excursion and have a blast!
Happy Cruising,
Michael
