As it turns out ferries emit CO2 in the air as part of their daily transporting operations. However since ferries are being operated at low altitudes, their CO2 effects in the atmosphere are very minimal compared to aerial transports where their CO2 emissions gives direct impact.
Ferries nowadays can also carry larger count of passengers which is good for the environment since their usage for transporting may not need more fuel as they used to.
Did you know that new ferries being built are now running on electric-propulsion systems that allows them to operate without any carbon emissions at all. This hybrid system would pave the way for modern ferries not to be a part of other mechanisms that causes air pollution.