There are. Details escape me but I think there is one that leaves from Pelican, cost around $50-$60 per person. Trip was around 45 minutes.
Forum: The Friendly St.Martin Island Forum
Agree about Divi Little Bay. Nice beach with very calm water. Some of the cruise ships offer excursions to Divi which include transport and lunch. Makes it easy, especially if traveling with couple small kids.
Forum: The Friendly St.Martin Island Forum