Not sure I agree with the earlier statement that most cards have no foreign transaction fees. Most do. Best to check around and obtain one that pays you back for all transactions as well as having zero for foreign transaction fees.

Always a good idea to exchange some funds for euros before departing. I always carry dollars and euros when visiting St. Martin. For some things having cash is the only thing that may work, such as the cost of a taxi. When dining on the french side always take time and review the menu to see if different prices are listed for dollars in one column and euros in another. This is rare, but it doesn't hurt to check. And, at the bottom of the menu there may be pre-printed language about paying an additional percentage if paying with USD. Of course, the first question you should ask would be "is the price the same for U.S. dollars and euros?" Taking the time to do this can avert some surprises when the check arrives. It is also wise to ask if a service fee is being applied (i.e., 4%).