From our experiences there is little to no reason to exchange US or CN dollars for Euros. We sometimes find it most advantageous to charge on the French side using a cc that does NOT charge a conversion fee and gives you the best daily rate for conversion from Euros to dollars but we generally only do this if the charge is significant. A day at Anse Marcel is one place we always use this approach since it tends to become a very pricey day there between chairs, food and drinks.
"Always keep your words soft and sweet, just in case you have to eat them."
Go to a bank on the French side and use your ATM card, sometimes your bank pays the fee or you pay like $5-10 service charge and get out a few hundred. You will get the current exchange rate ad euros in your hand.