Northeast of Grand Case off Saint-Martin, the snorkeling at Pinel Island is excellent. Pinel Island is part of a National Marine Park located off the coast of Cul-De-Sac. There is a crowded 10 minute ferry that provides access to the island. The most popular place to snorkel at Ilet de Pinel is the protected reef. There is a Reserve Naturelle's special underwater trail
If you plan to use your own boat, there are 15 permanent yellow mooring buoys. If possible, try to get to some of the other islands in the marine park to snorkel, especially Petit Clef or Green Key.
To reach the snorkeling on Saint-Martin by air, fly into Princess Juliana International Airport (SXM). Note, there is a $30USD departure tax collected at the airport.
In addition, there is a ferry called Great Bay Ferry that interconnects St. Martin with St Barts.
Overall, Pinel Island is the 1st most popular snorkel dive spot of all 4 snorkeling dives in Saint-Martin.