Careful: People often confuse CO2 emissions with NOx emissions when discussing ships.
The main concern with big ocean vessels is that they burn fuel "dirtier", as opposed to cars which--in most countries--are already subject to emissions standards and mandatory catalytic-converters etc. because people got tired of smog and acid-rain.
On a pure CO2 basis, ships may actually be the lesser-evil in terms of payload/distance, lack of alternatives, etc.
The main concern with big ocean vessels is that they burn fuel "dirtier", as opposed to cars which--in most countries--are already subject to emissions standards and mandatory catalytic-converters etc. because people got tired of smog and acid-rain.
On a pure CO2 basis, ships may actually be the lesser-evil in terms of payload/distance, lack of alternatives, etc.