[elihu]

For big container ships, that's a good question. I think it comes down to: low quality diesel fuel is cheap enough, and the regulatory hurdles would (presumably) be immense.

(Something else worth noting is that about half of all ocean shipping is just moving fossil fuels around. If we could just stop using fossil fuels, that by itself could cut ship traffic in half.)

I've wondered about the viability of battery-electric ships. Crossing the Pacific on a single charge isn't realistic given current battery technology as far as I know, but it seems like it might be possible to have transcontinental undersea power lines on heavily-used routes, and buoys at regular intervals with charging ports. So you'd have a ship travel for a day, then stop a few hours at a buoy to charge. The ships would probably have a small backup diesel just in case.

With nuclear, you could also imagine ship convoys. You have one small-ish ship with mostly just the nuclear reactor that trails electrical cables that other ships attach to. No need for every ship to have nuclear reactor.

One could even imagine a government operating the power-plant ships. For instance, let's say the U.S. Navy were to operate a dozen power-plant ships that travel a fixed route across the Pacific. Any ship could, for a fee, connect to the power-plant ship and be supplied power as it makes its way to its destination, and then switch over to batteries or diesel if the convoy route doesn't go all the way there. It seems plausibly feasible.