I've read that ammonia created from hydrogen & nitrogen using electricity from wind turbines is currently the predicted shipping fuel of the future. A less romantic way to harness the wind but rather more practical sounding. Less chance of being caught on a lee shore too.
This is the boring but correct answer, particularly as diesel engines can be refitted to burn ammonia until fuel cells are ready to take over.
However, to make it geeky and exciting again you could have autonomous floating wind and solar farms generating the hydrogen and converting it to ammonia.
The ships could then refuel at multiple points in the middle of the ocean (ideally without stopping) and so carry less fuel and save on the costs of fuel burnt transporting the fuel.
Energy density is a big issue. Bunker oil is nasty stuff, but weight/volume/energy is very hard to beat. And it's redicoulsly cheap, thanks externalities of local and global environmental impact/risk not being properly priced in. But ammonia-hydrogen at least approaches the ballpark of diesel fuel iirc (also much less concentrated energy - but a more "natural" replacement for toxic bunker oil, which is finally being banned due to the pollution from regular use and spills).
Some numbers here (Esp concerning energy density etc):
