I would not snorkel in Falmouth or English Harbour. No holding tanks! Instead visit the dockyard and watch the sun go down from Shirley heights.
North Sound is better and the water will be cleaner. Dont miss Nonsuch Bay anchor behind the reef just north of Green Island and you will get some of the best snorkeling in the Caribbean
Barbuda has some brilliant snorkeling but unless you are comfortable with real skinny water eyeball navigation you may find it a bit daunting. The charter boats are not allowed to go there so it is quiet!