East of Grand Case off Saint-Martin, the snorkeling at Turtle Reef is a well known for its namesake green sea turtles and hawksbill turtles.
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, Turtle Reef is the 3rd most popular snorkel dive spot of all 4 snorkeling dives in Saint-Martin.