Mel_mae...Welcome to the forums!!!
I have personally snorkeled in both St. Thomas and St. Maarten and found both to be fantastic! My favorite of the two was in St. Thomas...you can take a ferry to St. John and go to Trunk Bay beach where they have snorkel equipment for rent for $5. Then you can enjoy the beach and enjoy the underwater snorkel trail at your leisure. They have signs underwater that explain everything and it is really quite relaxing.
I have found that due to weather, taking snorkeling excursions can be hit or miss. If it has been windy, rainy or rough in the days leading up to the trip, the water may not be as clear as usual. It seems to me that Trunk Bay is more protected and it has always been crystal clear there for me.
Just my two cents.
Good luck and have a wonderful cruise!