For the majority of our past trips to St. Martin and Martinique, we took out some euros at an ATM when we arrived but primarily used a credit card. For our most recent trip to St. Martin in December, we didn’t make it to the ATM for the first few days and never had any issue paying with USD or a card. We eventually decided to just skip the euros completely and were fine. We stayed in Orient, so most places seemed OK with either a card or USD. Our credit card doesn’t charge a foreign transaction fee, which made things easier. We did occasionally have to ask to be charged in euros, though. Some places assumed we preferred USD. I’m not really sure if this impacted the exchange rate, but I figured choosing euros couldn’t hurt. The only time I wished I had euros was to tip in cash. We were able to tip in dollars, but I would have preferred euros since that's what we were paying in for the meal on the credit card and I thought it might have been easier for the servers to eventually use.