Agree, always let your credit card company convert currency.
You never let the ship do they conversion - they use a mid rate and then charge 3%. Always select the option to let your creditcard do the conversion. I always highlight this is bright orange highlight pen too just to make sure it stands out!
Get a Nationwide card just for travel abroad - they do not have charges for conversion, then have your card charged in the ships currency. Never let the ship do conversions they make more money out of you that way .