We were just in SXM last week and we did the Tango to Shoal Bay. It was a great day. You arrive at one beach (Cove Bay?) and take a bus to Shoal - the ride probably takes 15-20 minutes so you do get to see a bit of the island. Once at Shoal you get chairs and lunch. They have noodles and snorkel gear available for use (included) - we snorkeled and had a grand time. The beach is gorgeous and lunch was very good as well. I highly recommend the trip.