I’m pretty sure this doesn’t hold up. Airline travel is pretty terrible for carbon emissions, but it’s over in 6-7 hours while the ship has to keep you alive (and has to haul around the supplies and equipment to do that) for a week.

30 seconds of research turned up these numbers: kg CO2 / pax-mile: QM2: 0.43 long-haul flight: 0.257 theguardian.com/travel/2006/… A more spartan boat than the QM2 might cut that figure in half, but more modern airliners can beat the industry average as well.

The last time I went across the pond, I did it in an LCC-configured 737 MAX 8. Getting the threefer of narrowbody jet, the most modern generation of engines, and seats jammed as close as knees allow, was probably the greenest way across the Atlantic available with today’s tech.

But big planes are nore fuel-efficient than small planes if they're full...

Not strictly true, there’s a big discontinuity when you add that second aisle.

(Also, there aren’t any widebodies out there with geared turbofans, yet.)

Sure, but IIRC the 380, 787, and 350 still have lower fuel burn per seat than the 737 MAX and 321.

Definitely not on the 787. A350 is very close. An A380 might actually, in a high-density configuration that no A380 operator has ever come close to flying in revenue service.