In Barbados look for a snorkel excursion that takes you west from the port to the beaches along the southern side of the island. There is a boat called Harbormaster that does a cruise, snorkel beach , fun and sun type thing. There are usually catamaran
trips as well.
In Martinique look for an excursion that does one or two stops that includes snorkelling at the bat caves.
In St Martin it is said the best snoekelling is at Pinnel Island. I found it to be a fun trip, but the snorkelling was just ok.