I did this tour after being told snorkelling was good, and we were disappointed.
Bus to french side followed by a 5 minute boat ride to Pinel Island. There is a small food place and not much more on the island. Beach for snorleling is not much at all. Hardly held everyone on the tour. Snorkelling was ok at best. A lot of one kind of fish and little coral or underwater color.
I can recommend Dawn Beach as a great BEACH but I cannot comment on the snorkeling there.