That’s hardly low carbon, container ships alone emit almost 20% of the CO2 we are putting into the atmosphere. And Cruise ships emit almost 4x as much as air travel per passenger.
You are correct about cruise ships, although it's more like 2-3x.
[1]: https://phys.org/news/2018-12-cargo-ships-emitting-boatloads...
Or put it another way - if all that cargo going by sea went by air C02 emissions would be 100x more.
Obviously saying sea travel is massively more efficient doesn't obviate the need to reduce that as well.