For that matter, all transportation combined is only 24% of greenhouse gas emissions. Tackling utility energy would be a much larger target, and easier to do. Nuclear power works.

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!