North of Grand Case on the northcoast of St Martin, the snorkeling at Anse Marcel offers calm waters in a very protected bay. There is an interesting rock formation in the bay called Rocher de L'Anse Marcel which is a good destination for very advanced snorkelers.
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, Anse Marcel is the 6th most popular snorkel dive spot of all 10 snorkeling dives in St Martin.