The official currency of St. Maarten is the Antillean (not the Dutch) guilder, which is pegged to the U.S. dollar at 1.79. In practice, the dollar is used predominantly and a visitor is unlikley to ever see a guilder. The currency of St. Martin is the euro, the same as in Europe, but again dollars are accepted everywhere, although euros are much more common then guilders. The value of the euro varies (currently worth more than the dollar), and while some establishments will give 1:1 echange, others will not.
Major banks include ScotiaBank, RBTT (Royal Bank of Trinidad and Tobago), and Winward Island Bank. ScotiaBank is affiliated with Bank of America and BOA customers are not charged a fee to use the ATM.