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.