This paper explores energy consumption and carbon dioxide (CO2) emissions of a coastal transport system envisioned as a substitute for road transport. Past work investigating the economics of coastal passenger transport via high-speed small passenger dolphin-type ferries has shown that the most profitable sea itinerary is achieved when fewer mid stops and the highest ticket price is applied. Road and maritime distances, fuel consumption and CO2 emissions were calculated for typical passenger cars and marine vessels. Although a ferry is a massive consumer of energy and emitter of CO2 compared to a single passenger car, the capacity of a ferry is much larger. The reduction that must be achieved in the number of cars per ferry trip in order to overcome the increase in CO2 emissions due to the ferry was estimated for various occupancy levels up to a capacity of 250 persons. Depending on the number of mid stops, high occupancy levels of the ferries must be achieved in order to realize a negative CO2 contribution to the atmosphere.