The ship I would most like to cross the ocean on:
https://www.starclippers.com/us-dom/our-fleet/royal-clipper.html
I don’t see how this could possibly be true, nor does the article imply it. The mistake in the article is comparing the kite to a sail. This is not even considering the fact that the ships in question don’t have a keel, so that alone makes sailing upwind, in the manner of a sailing yacht, impossible. The thing that allows a sailboat to sail upwind is the combination of the fact that wind pressure pushes the sails into airfoil shapes that act in concert with the keel to move the boat forward. None of that is operative here.
The kite closely resembles what on a sailboat is called a spinnaker, a large sail that balloons out in front of the boat and is used for sailing more or less downwind. In fact a spinnaker is sometimes called a “kite”. But a spinnaker can’t take you upwind, and neither can the kite. If you’re going upwind you tighten up and close-haul the main and the jib and let them act as airfoils.
I was looking through the Skysails brochure and I need to correct myself here, at least according to their claims. The key factor – and the one that contradicts my spinnaker analogy – is that a large number of lines are used to control the kite, allowing it to be shaped under wind tension as circumstances require, including the ability to assume a winglike airfoil shape. On this basis they claim that it can, in fact, pull the ship upwind as much as 50 degrees from the wind direction. But then they hedge a bit and say that “in practice” it’s more like 70 degrees, which is getting pretty close to just a crosswind. They further say that the most efficient courses are between 120 and 140 degrees from the wind direction.
So it looks like I was wrong about the upwind part because of the ability to shape the kite into an airfoil, but it also looks like their practical upwind performance isn’t very good and quite far from that of a sailing yacht (by about 30 degrees), and they’re predominantly useful on downwind courses.
The company does that does boats as well 
Application of an Automated Kite System for Ship Propulsion and Power Generation | SpringerLink
Give a couple of examples of what those passengers could have done if they didn’t go on that cruise and that:
a) Is anywhere near comparable
and
b) isn’t also considered an environmentally unfriendly choice
A massive cruise ship is a consolidator of 5000 people into an easily packaged form that allows an easy cheap shot about the pollution from it. OMG the sewage! The carbon! As I’ve said above I don’t doubt cruise ships are inefficient. But the standard hatchet job article on them (as linked in the OP) always describes the pollution produced by a ship without any comparator; as if those 5000 people wouldn’t exist if they weren’t on the cruise ship. When actually there should be a comparison to (at the very least) the pollution produced by a ship vs the pollution that would be produced by 5000 people who stay home. Guess what - 5000 people at home produce a metric shit tonne (pun intended) of sewage, carbon etc.
And then people who don’t go on a cruise probably vacation elsewhere - so the more realistic comparison would be the pollution produced by a ship vs the pollution produced by 5000 people going to a resort somewhere, which would produce more carbon than them staying home.
You say I should give a comparator that “isn’t also considered an environmentally unfriendly choice”. No I should not. The 5000 people who don’t go on a cruise ship are actually quite likely to do something that isn’t also considered an environmentally unfriendly choice.
I was figuring on some combination of mostly carbon tax pricing (as Dr.Strangelove mentioned) and a little flygskam accomplishing that IF there’s a “spot” where an ocean liner can be fast enough and comfortable enough to have just significantly less enough carbon emissions than flying that companies would bother to offer the service and travelers would bother to take it.
I don’t think emissions would be the problem. The problem is as LSLGuy says, time. On emissions, you could stack a large ship with enough people, in (very) modest comfort to make the exercise carbon efficient. But it’s the time…
container ships (efficient because speed is minimized and occupancy maximized)…
Bad choice of example. Container ships are the greyhounds of the seas. 25+ knots.
You make some good points. Certainly the sewage would still be generated.
But likely not the sulphur.
I don’t think that’s true any more:
Slow steaming was adopted in 2007 in the face of rapidly rising fuel oil costs, which was 700 USD per tonne between July 2007 to July 2008.[4] According to Maersk Line, who introduced the practice in 2009 to 2010,[5][6] slow steaming is conducted at 18 knots (33 km/h; 21 mph).[1] Speeds of 14 to 16 kn (26 to 30 km/h; 16 to 18 mph) were used on Asia-Europe backhaul routes in 2010.[7] Speeds under 18 kn (33 km/h; 21 mph) are called super slow steaming .[1] Marine engine manufacturer Wärtsilä calculates that fuel consumption can be reduced by 59% by reducing cargo ship speed from 27 knots to 18 kn (33 km/h; 21 mph), at the cost of an additional week’s sailing time on Asia-Europe routes.[8] It adds a comparable 4 to 7 days to trans-Pacific voyages.[7][9]
It probably varies based on the daily price of bunker fuel, though.
The sulphur thing is a furphy. Since a year or two back ships can no longer use high sulphur fuels - or have to use scrubbers to remove the sulphur before use. This is far too inconvenient a fact for most anti-cruise ship polemics to bother mentioning. They still do the “one cruise ship is equivalent to every car in Europe” thing because it’s just too good a line not to use.
Yes, some such have now been updated in this respect and instead now complain that scrubbers remove the sulphur and dump it into the ocean OMG crank the panic up to eleventy! They probably send dihydrogen monoxide overboard too!
The problem with burning sulphur is that it creates sulphur dioxide in the air. But dissolved sulphur in the ocean is part of a normal biological cycle, and how the sulphur got into the fuel in the first place.
I don’t think that’s true any more:
What is now true is that container vessels sometimes slow steam. Not that they now always or even often do.
Neat! That their system is exactly what I had in mind makes me wonder if I’d already heard of it…
When sailing close to the wind, not only is the delivered power low, the control is inherently unstable (as anyone who has flown a kite will remember). Since it’s an expensive system “keeping control of the kite” is a critical factor. I don’t know how good SkySail got at that: it’s always been a consideration for other designers.
The Wiki article says that “nearly all” lines are doing it, and has some cites:
“We continue our slow steaming and fuel-saving activities,” said Nils Smedegaard Andersen, Maersk’s group chief executive, who has put “basically all ships on slow steaming.”
Granted, the article is from 2010, but I’m not sure anything has changed since then.
Creating clouds, and reducing global warming…
Sulfur Dioxide: Its Role in Climate Change | Science Mission Directorate (nasa.gov)
According to the shipping reports, compared to 20 years ago, yes, they characteristically slow steam. Always or often. Did you have a particular route in mind? That would be interesting.
It’s amusing that I sometimes find myself defending cruise ships against the more exaggerated complaints against them, given that you couldn’t drag me onto one without a bulldozer and I wouldn’t piss on one if it were on fire.
As a matter of curiosity how do you know about ships? The crossover between those that know something about ships and those that call them “boats” is small* 
*outside the US
I can’t resist one of my favourite aphorisms.
Q. What is the difference between a ship and a boat?
A. A ship can have boats, but a boat can’t have ships.
The Oz navy’s new Arafura class patrol boats are an amusing anomaly. At 80 metres they are hardly boats anymore. They are splitting the difference and calling it them vessels. Submarines are of course always boats.
IME in the industry a ship can be called a ship but the real pros call them vessels.
It’s funny how important is language. I took one of my juniors down onto a ship last year and the crusty old bastards clearly regarded her as too young and probably too female to know anything but the respect went up several notches once she started using the right language.
One actually said “well least she knows it’s called a vessell”.
I was about to argue with you but then I realized you were talking about the SkySail, not conventional sailing. I’m sure you Aussies know all about sailing! But I’ll say anyway – because it made such a deep impression on me when I was first learning to sail – that from a purely visceral point of few sailing close to the wind in a substantial sailboat driven by a fresh breeze is the best and most exciting kind of sailing. The boat heels over dramatically and you can literally feel the tremendous power of the wind, especially in rough water as the boat leaps through the waves, with the spray coming over the bow.
And that’s the difference between a ship being towed by a kite and a real sailboat.
Either you dont know what “furphy” is or you are being rather disingenuous. There were several solid cites about it up there by the OP. But I can see that could be outdated.
A furphy is Australian slang for an erroneous or improbable story that is claimed to be factual. Furphies are supposedly ‘heard’ from reputable sources, sometimes secondhand or thirdhand, and widely believed until discounted.
What you are saying is that The sulphur thing is outdated now, for at least a year. Ok, sure. But those cites are neither erroneous or improbable. Outdated, maybe. June 4, 2019 was the date on one.
Now, I totally believe you but a cite would be a nice thing for people that dont.