Ok fair enough. If we stipulate that, then we just need to regulate these emissions because they are getting a free ride on externalities. We just need to say '

Ok fair enough. If we stipulate that, then we just need to regulate these emissions because they are getting a free ride on externalities. We just need to say 'if you wish to dock at our port, you must have a boat that emits less than x'. The market will then adjust by building ships that are cleaner. Cost of a cruise will go up by a few bucks, problem solved.