In Barbados, we have used Tiami Catamaran sail/snorkel 6 times. They have beautiful cats and great crews. They take you to the same snorkel area, but it's beautiful, so it's fine with me.
In St. Lucia, we have always booked through the ship, except for our last visit and we took a cab to Pigeon Island. It was much less expensive through the ship.
In Antigua, we just negotiated with a driver outside of the terminal area. It was $25/pp and we got a great tour.
In St. Maarten, you can take the water taxi to the beach near where you dock. We used the tikihut the last time we were there. There's great shopping right at the port, but loads of people go to Marigot. There's also Maho Beach and Orient Beach. The cab fare is reasonable. Orient Beach, you will have to purchase a drink package to use their restrooms, some will include a chair, too.
St. Croix, haven't been there in ages, so not sure who to use there. However, the snorkeling when I was there was really good.
Hope this helps a little.