Crew ships and transport ships are pretty nasty. In the summer my island town of 15k gets 4 to 9 crew ships visiting. That is like and extra 4 to 9 million cars driving around all day. Sure, on a global scale the impact of a single ship doesn't account for much. But, it is orders of magnitude more than my driving. The focus of personal vehicle emissions is misplaced if the goal is to reduce overall emissions quickly. The focus should be of the worst emitters first.