Necessary? No. U.S. currency is accepted just about everywhere from my past experiences. But, I do typically bring a small supply of euros to SXM for dining purposes for those situations where the prices in USD are set much higher than in euros. You will only encounter this on French St. Martin, and not on the Dutch side. The last time I checked one U.S. dollar is worth .84 euros. Kind of weak IMO. It's been much higher than that in past years. So, if you encounter a restaurant in Marigot or Grand Case that, for example, applies a 30 percent surcharge if you pay in USD, you could be better off just paying in euros or using a major credit card. Menus usually list prices in both currencies, or will have small type print at the bottom which will show the amount of exchange. If nothing is displayed I usually ask this question before ordering. In addition, if you plan to shop on the French side for groceries I prefer to pay in euros rather than trying to figure out if I'm being overcharged if I pay in USD. On the Dutch side I use U.S. currency for everything I do.