What if it was replaced by boat travel?

You could take a boat. It takes longer but you said it's not an emergency.

yeah i'd love if ocean liners were a thing again. afaik there's only one, and it's a ritzy and touristy affair

Ships are worse than planes.

https://theicct.org/marine-cruising-flying-may22/

in this theoretical world without commodity air travel, we'd have ocean liners fit for purpose. right now the only one i know of is effectively a cruise ship payload stuffed into an ocean liner hull. based on the numbers for cargo favoring sea over air, it seems reasonable that liners _could_ be more efficient than flight

The huge amount of time saved with plane vs boat makes the former the superior option. There's no way a ton of CO2 is worth the comparative QALY loss you get from five days in a boat compared to the other things you could be doing with that time (if you're on a properly efficient liner rather than a super inefficient modern luxury cruise liner). Even the extreme upper end of CO2 externalities don't go above $1000/ton with most reasonable estimates below $100/ton.