Maritime traffic dropped significantly during mid-2020 but has largely recovered. For example, Singapore's stats for Feb 2021 are basically flat year-on-year for container throughput.
Super-ships emit ~5g_Co2/Ton/km on average[1], while railway is 2-35g, air transport is 700-3000g and truck transport is 100-2000g.
Surprisingly, the bigger the boat the more effective. We tend to have a bias against larger machines, but often they can be the cleanest in proportion.
There's nothing surprising about large ships being more efficient. But a big issue is that many ships burn dirty fuel. Apparently there are no fuel standards in international waters.
As for the comparison to trains, it matters a lot whether you're talking about diesel or electric trains. Most train lines in Europe are electrified, and as electricity production switches to solar/wind, it may actually end up being cleaner than ships. (Although work is also being done on making ships cleaner. But new international laws are probably needed to get everybody on board.)
The real issue, though, isn't no much whether the transport happens by boat or train, but that it happens at all. The scale of global shipping is this big because everything is produced on the other side of the world. Big ships make that transport more efficient, but a more egalitarian global economy that didn't create incentives for companies to seek out every low-wage country and tax haven, would make local manufacturing more attractive and reduce global shipping.
Isn't the problem that ships are burning heavy oil instead of diesel or similar? CO2 isn't the main worry afaik, it's the rest that gets blasted unfiltered into the atmosphere and the left over sludge that gets illegally dumped into the sea.
The upper limit of the sulphur content of ships' fuel oil was reduced to 0.5% (from 3.5% previously) - under the so-called "IMO 2020" regulation prescribed in the MARPOL Convention. This significantly reduces the amount of sulphur oxide emanating from ships.
5g per km-tonne adds up to a lot though doesn't it.
"Maritime transport emits around 940 million tonnes of CO2 annually and is responsible for about 2.5% of global greenhouse gas (GHG) emissions (3rd IMO GHG study)."
And it makes sense if you think about it - as you scale up, the ammount of fuel you can carry goes up roghly by cube while structure mass, forming a shell effectively arround the fuel, goes up by square of size of the rocket.
Similar things for air resistance - as you scale up yuour rocket the front part creating the most drag will scale more slowly than the volume of the rocket that goes to fuel, payload and structure.
No wonder Starship is already at 9 meters of width and 18 m has been mentioned as a possible future upgrade. Its already bigger than the massive 66+ meter high medieval watchtower in my home town yet it can fly to sub orbital speeds without its first stage booster (which is even bigger)!
For pressure vessels like rockets, the mass of the structure and the mass of the fuel scale together as far as square-cube reasoning goes. The surface area of the structure scales with the square while the volume of the fuel scales with the cube, but the thickness of the cylinder walls must also increase, so you end up with cube vs cube.
Possibly road conditions and distance. A truck moving goods around a city will be much less efficient (stop-start traffic) than a truck moving goods over a long distance between cities (likely mostly highway).
