Why single out ships? Maybe we could just tax emissions such that the activity does not matter.
The problem with ships is that once they enter international waters they can do whatever they want.
Which is exactly what they do, burn bunker fuel outside territorial waters and switch to cleaner fuel when they approach a country (because they are forced to).
There is simply no enforcement outside territorial waters, and if anyone tried to, they'd be immediately condemned by everyone else.
Is it possible that you are a member of a group of individuals that do not enjoy nor participate in cruises and therefore do not give anything up by calling for their ban? You can feel good about it without personally sacrificing anything and instead forcing others to make that sacrifice.
How would you feel if it were the inverse and others were calling for a ban on an activity you enjoy?
It is very easy to call for a ban on something that you personally do not enjoy because you lose nothing. In fact a majority of the world's population likely does not enjoy cruising, just as a different majority does not enjoy snow sports, water sports, car racing, motorcycles, horse racing, skydiving, rugby, rock climbing, etc..
There is some form of ethical or safety concerns for each of those but the world would be a very boring place if we banned them all.
Of course, it could be that the C-H bonds contribute much more to the energy released by burning the fuel than the C-C. But this doesn't seem to be case. Here's a paper in favour of switching to lighter fuels for ships: it argues that the total supply chain CO2 contribution would be higher (it takes energy to refine those fuels), but only by 1%.
I think that "bunker fuel is really bad and ships switching to lighter cleaner fuels is important for the environment" is a seductively easy but bad target for carbon reduction. There are way better places to spend our efforts.
[0] https://www.tandfonline.com/doi/pdf/10.3155/1047-3289.58.4.5...
The easiest, most economically efficient, most productive way to do it is to tax the C content of fuels. Use the monies raised to subsidize green energy.
Banning is an indiscriminate club that does a lot of collateral damage.
The regulations we have now (no high-sulphur fuels in cities, no leaded gasoline...) are better than taxing only CO2 emissions, even if they're not perfect.
The problem with banning is its inflexibility. For example, let's say you have a bunker-fueled emergency generator for a hospital that you use only when the power goes out. The pollution it emits is negligible. But by banning bunker fuel, you'll have to replace the emergency generator, at considerable expense and environmental cost.
With taxes instead, you:
1. bunker fuel will only be used when there's high value for it, and you won't have to add a plethora of exemptions into the regulation
2. have a revenue center for the government instead of a cost center - revenue that can be used to subsidize green initiatives