What continent is the island of St Maartin part of?

ummmm.... The island isn't really part of any continent, unless we get another big freeze and the seawater level drops REALLY low. But then, all of the Caribbean islands would be part of a greater American continent that would stretch from southernmost Agentiana to northernmost Canada.

here's a link showing maps of continental shelves around the world: http://www.edc.uri.edu/lme/clickable-map.htm

another reference (including how the island is divided between two nations):

http://www.worldatlas.com/webimage/countrys/namerica/caribb/stmartin.htm