In St. Maarten, we did the TikiHut excursion. The operators were fantastic and the best part was you could stay as long as you like. They picked us up and took us to their dock. If you needed snorkeling instructions, they would give them to you. They had a pool like area where you could practice. The snorkeling was very good. There was a restaurant/bar area. We booked through the ship, but it wasn't very expensive. They would also bring you back to the ship or take you to the shopping area where you could catch the water ferry back to the ship. Here is the link http://www.snorkelstmaarten.com/about.htm
They cannot book independently if it is offered through the ship, but if it isn't offered, they can take you independently.
In Antigua, we walked out of the terminal area and there were a bunch of local tour guides waiting. We paid $25/pp and he took us all over the island. After a few stops, he took us to a nice beach with a restaurant/bar and let us stay for a couple of hours, then took us back to the ship. It was a great tour. We also did a snorkeling tour through the ship, but I wasn't really impressed with the snorkeling, way too many jelly fish. It may have been the time of year (October), but when we went back, I didn't want to try it again, so we took the tour.