"One giant container ship pollutes the air as much as 50 million cars. Which means that just 15 of the huge ships emit as much as today's entire global 'car park' of roughly 750 million vehicles. Among the bad stuff: sulfur, soot, and other particulate matter that embeds itself in human lungs to cause a variety of cardiopulmonary illnesses. Since the mid-1970s, developed countries have imposed increasingly stringent regulations on auto emissions. In three decades, precise electronic engine controls, new high-pressure injectors, and sophisticated catalytic converters have cut emissions of nitrous oxides, carbon dioxides, and hydrocarbons by more than 98 percent. New regulations will further reduce these already minute limits. But ships today are where cars were in 1965: utterly uncontrolled, free to emit whatever they like."
According to Wikipedia, 57 giant container ships (rated from 9,200 to 15,200 twenty-foot equivalent units) are plying the world's oceans.