Longer term I think we'll also use synthetic liquid hydrocarbon fuels manufactured through the Fischer–Tropsch process. People have proposed bringing back nuclear powered merchant ships but that's a non-starter for cost and security reasons. We can keep the nuclear plants safely on land and use them to make carbon-neutral fuel.

That is insanely inefficient however. It only makes sense for transoceanic air travel, probably supplanted by synthfuel from biomass, but in the long term we will see ships powered by batteries. Far far more efficient.