why not alcohol? It's waste gasses are not too bad. The whole CO2 BS needs to go away. I can get the not wanting hydrocarbons because of the nox and sox but CO2 is a given for even you being alive.
What if they had a barge, a large ocean going one, it had a lower profile, hauled stuff, it's entire deck top was solar panels. Morning time, it charged up a little bit at night using wave action, to lift an anchor, then used the panels to push a motor on a propeller to drive it towards whichever destination it was heading. when night came, it dropped anchor to sit where it ended up at, unless the prevailing winds would ultimately push it towards it's end destination even further, at which time it'd just keep drifting naturally.. While sitting it could use wave generators to make electricity by the up and down on the waves to charge a battery. come next day, do it all over again, during bad weather / cloudy days, it'd just charge it's battery, you could put wind generators on it too, not stupid tall ones but smaller ones along the deck to gather any wind that might be going too. Perhaps every 10 miles or so you could have a small platform it could anchor / moor to, like stopping points, or every 50 miles, whatever works, if needed or for emergency docking. have enough of these on a planned route to put these barges between countries on planned shipping routes. You could have enough of them running that if it took a few days more to get somewhere, no big deal. It'd essentially burn zero fuel except what it made on it's own.
when it gets wherever, unload, put new stuff to go back and send it back. put a few dozen people on it, or even just a few with some rockets to fight off pirates, a keg of beer, some weed, a box of doritos, and you are good to go ! Throw in a fishing pole and they are loyal to the end! Catch dinner, and perhaps some extra fish to sell when they get back in port, a bit of a trip bonus!
Hell sign me up, Ill be the first one to test pilot the HMS UnicornPhart.
Aaron