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.