Hear me out. The thing with cruise ships is that they sometimes relocate from North America to Europe operations and vis versa. Those cruises don't get tourists because the whole point of a cruise is to visit different places, no one wants that trip. But the relocation is going to happen anyway, so they sell tickets super cheap (and it's stripped of entertainment like comedians, shows, etc). Because the relocation is going to happen anyway, you're not exactly adding to any carbon footprint. You'd have to Google what exactly they're called.

But I get you if you want to sail. That does exist too.

It's generally really hard if you have no experience. But if you're willing to pay, maybe. Check this out: https://hitchwiki.org/en/Hitchhiking_a_boat

I mean, Cunard does transatlantic sailings with the QM2, but your environmental impact for doing so would be high. Your impact would be far lower (per kilogram) if you did travel passenger on a cargo ship.

Looks like the main options are the things you've already ruled-out:

• Lowest impact travel: passenger on a cargo ship.
• Zero-emissions vehicle: £6000 trip on a sailing boat, but any normal-sized boat is going to expect everyone on board to take shifts.

Maybe you can find a "tall ship" that's big enough to have passive passengers (example), or pay the small boat to bring a higher ratio of paid crew to let the passengers sleep.

Like a transatlantic cruise?

As others have said private boats are probably the best bet, but there are also sailing cruises which cross the atlantic, I believe the Alexander Humboldt stationed in Bremen makes that trip once a year. There are also other ships that I forgot the name of. It is really expensive though

Lots of sailors look for crew of varying levels when crossing with personal boats. Lots of work aboard for tall ship experiences as well. We live in a place where there are loads of sail boats on the east coast of North America and folks are looking all the time. Head to a marina and start asking around. Find sailing groups on facecrack or any social media and advertise yourself. Many will require experience but many will train you as well.

I think most sailing ships do expect all passengers to be capable of crew duties to some degree. Especially on an arduous and long journey like across the atlantic

https://www.59-north.com/ take you on as crew for different lengths of legs. I think they usually go back and forth with two boats per year.

I'd go for something smaller first and gain experience.

Royal Caribbean has a couple of one way routes from the us to Europe.

Some shipping companies will also rent cabin space for civilians to cross the atlantic.

Feeding and housing you for the multi day trip will outweigh any emissions reduction from travelling by boat instead of by plane