You can also turn water + atmosphere into synthetic fuels, such as methanol or even kerosene. Such fuels can be used without modification to existing ships. Alternatively, you can just electrolyze water and get hydrogen, which can also be used if you can deal with the lower volumetric density.

I wonder if that'll become feasible in the near future. I can't find any up-to-date articles. But the ones from one or two years ago seem to indicate 1kg of green Ammonia from solar farms in Egypt costs 4.50€. While a liter of crude oil was something like 13ct. So I think oil needs to become a lot more expensive to make this happen.

This is ...wishful thinking. As in I really wish it were true. And it is kinda, but you're talking about 0.0001% of the oceangoing fleet and nearly as little for wind farms doing this.

The infrastructure for this kind of fuel isn't there. In fact, there is an intense battle between ammonia, hydrogen, biofuels and LNG (thus far in the lead) for what will replace fuel oil/gas oil. Until the infrastructure is built of something very, very seriously constrains supply, we are stuck with carbon for at least 20-25 years no matter how hard we pivot tomorrow. Not saying we shouldn't pivot, but that's the commercial reality.

But at this point alternatives are few on the ground and consist mainly of small ferries with batteries or huge duel fuel (LNG) containerships, and there are less than a couple of dozen of these in the world now.