The Dutch and the French side has different entry requirements:
For the Dutch side, according to
www.st-maarten.com,
U.S. citizens require one of the following:
A valid passport and a return/continuing ticket.
A birth certificate with raised seal and photo identification and a return/continuing ticket.
Naturalized citizens must show an original naturalization certificate with photo identification and a return/continuing ticket.
Permanent Resident (Green Card) holders must have a valid passport and a return/continuing ticket.
A Valid Passport is required for Everyone Effective January 8, 2007.
For the French side, according to
www.st.martin.org
Official Requirements :
French or EU citizens : national ID card or valid passport.
For the rest of the world : valid passport and return or transfer ticket, no visa is required for stays up to 3 months.