I would first call CAnival and see if they will reverse the charge for the excursion. If they don't you can dispute the charge with your credit card company.
Just make sure the next cruise, you re-check the charges before you get off the ship, much easier taking care of it then, best of luck. For some reason, it may take Carnival a few billing cycles to get the charges off, funny how quickly they can add them on, but takes forever to get them off??