It's generally not necessary to change your money in St. Maarten/St. Martin. All banks now have ATMs that accept international cards. Remember, though, that they may issue just dollars or just euros. On the Dutch side, try RBTT or Windward Islands Bank, both of which have several branches on the island. On the French side, try Banque des Antilles Françaises or Banque Française Commerciale. MasterCard, Visa, and American Express are accepted all over the island, Diners Club and Discover on occasion.
Legal tender on the Dutch side is the Netherlands Antilles florin (guilder), written NAf or NLG. At this writing, US$1 was equivalent to NLG1.82. On the French side, the currency is the euro. The exchange rate at this writing is EUR 1 to US$1.35.