The state of biofuel today leaves a lot to be desired as far as the ability to scale without causing other environmental disasters, and many of them aren't even carbon neutral.

As for nuclear cargo ships -- I'd love to see them, but the liability and political hurdles are insane. You'd probably need a really big carbon tax before we'd see them.

Even without nuclear, container ships don't have major weight restrictions (they already weigh a hundred thousand tons), so you can fuel them with anything that will catch on fire. Make carbon expensive and they'll just buy whatever is the next cheapest -- sawmill residue, seaweed, whatever. A lot of the difficulty with "biofuels" is the process of making them into oils that can directly replace petroleum in road vehicles, but large ships don't need that. A large furnace like that can burn unprocessed solid biomass.

Although, burning biomass in ocean-going vessels is not necessarily a net positive. If, for example, they start burning sawmill scraps which were previously being pressed into Ikea furniture, you're just replacing one carbon emission with another. Biofuels are complicated; you have to be careful not to offset sequestration, as well as being cognizant to other emissions generated both in the process of generation and in burning them. Because of the tangential effects, some biofuels emit more carbon than oil!