Not saying boats are perfect or that every company that runs them are angels! But you have a lot of head-room if you start off 100x better kg per kg.
Because boats are not perfect doesn't change the original point:
A 20% saving on emissions for aircraft and talk of 'having more space for cargo' is missing the big picture - air travel is a massively subsidized luxury where the speed of travel comes at a massive energy cost.
> Sending air mail is a luxury now?
Yes - either wait a bit longer if you need to send a physical thing - or if it's just a message then do it electronically.
Yes - perhaps an email isn't as 'nice' as a real letter, but isn't that the definition of luxury?