Basicly ANYTHING can be made on the ocean (including cell printed human livers - and of course fuel) as long as you have the two universal basic ingredients SPACE to put a factory there and ENERGY to run it .
On the ocean you have both in abundance and economic supply…
The source of carbon for fuel production can be the CO2 from the air, what makes the fuel carbon neutral …
The good thing is that the plug in into the existing system is seamless - fuel to supply the cities already comes in tank ships from the open sea and is fed into the distribution channels in the port - produce it synthetical in floating factories is just the next logical step beyond the usual oil fields.
If it is a better way to do it with a chemical factory or use the way over special microbes or algea to do the molecular assembly of simple components mostly C and H to longer molecular chains that form the liquids that we call and use as “fuel” is yet to see and test out in pilot projects.
What is clear by now is that we need a paradigm shift that steers away from anything “fossil” in our energy supply chain to make life on the planet for 7,5 billion humans feasible.