Ships are pretty good emissions wise because if the sheer amount of cargo they carry, even with burning the absolutely worst crap they can
As we ramp down fossils we this kind of thing would enable eg international agreements to limit and progressively lower shipping sector co2 credits over time.
We need to do this for all co2 emitter sectors since all the slices of the pie are fairly small so focusing on eg top-3 doesn't solve the problem.