For those mega cargo ships, why is civil nuclear power vessels are not a thing? Seems they would massively reduce the footprint while providing better clearner power.
Because it's all about cost. Those 400m container ships are ran by a low cost crew of 15-25 with some 3rd party maintenance personnel sprinkled in. Try fitting a more expensive fuel and more maintenance into that. On top of all safety concerns.
For ocean crossing vessels they are more closely looking at e-fuels and hydrogen since that gives the same benefitd at cheaper cost with not a too large change in the shore side infrastructure.
Servicing marine nuclear reactor units requires very special skills and is expensive. So is the initial capital cost. Servicing large marine diesel systems uses labor that is much more available globally.
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.
Cost, safety, scale (there are 80,000+ commercial vessels), reliability, crew training and costs, etc.
Combustion's highly proved, reliable, and safe technology. Range is sufficient for transoceanic transport. Emissions (both CO2 and other factors) are an issue, but could be mitigated with synthetic fuels, possibly biofuels (though even shipping would strain capacities required).
For ocean crossing vessels they are more closely looking at e-fuels and hydrogen since that gives the same benefitd at cheaper cost with not a too large change in the shore side infrastructure.