West of Grand Case off the northwest coast of St-Martin, the snorkeling at Happy Bay is popular for plentiful reef fish. The best parking is at Friars Bay and then you'll need to walk over the cliffs to the scenic Happy Bay beach. There is good snorkeling to be found over the rocks on the north and south ends of the beach.
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, Happy Bay is the 8th most popular snorkel dive spot of all 10 snorkeling dives in St Martin.