TAE, helion energy and ZAP energy are 3 well founded companies (combined $1B+) aiming at building ~50MW generators based on non-tokamak fusion.
The generators are small enough to fit on a truck and could easily power the biggest container ships.
You've also got that ability without nuclear inputs, from conventional renewables (solar, wind, geothermal).
Those fuels can be far cleaner than bunker fuel, and a diesel or kerosene analogue burning in a well-tuned ICE or turbine would have limited negative effects. The marine-propulsion pollutants of most significant concern, aside from (fossil) CO2 are particulate, NOx, and SOx emissions.
All are far more prevalent with heavier fuels, and are concentrated largely along shipping lanes far out to sea, other than at ports. These tend to settle out / mitigate reasonably quickly, and I believe their long-term ecological impacts are fairly minimal.
The problems with nuclear marine propulsion are many, not the least of which are the 100--200 shipwrecks logged every decade or so, though you can add to that risks of piracy or terrorism involving nuclear propulsion. Keep in mind that present commercial fleets are on the order of eight thousand vessels. We've had experience with only about four hundred nuclear-powered vessels, virtually all military ships, of which about 170 are presently operational. The exceptions are three demonstration commercial vessels, the Otto Hahn, Savannah, and Mutsu, all of which failed to prove feasible, and a handful of Soviet-era icebreakers, now operated by Russia.
Fusion itself remains both technically and economically infeasable for any foreseeable future.
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I am just providing pointers so people could have a look and juge by themselves. I was pretty convinced by Helion and ZAP, less by TAE.
Because we are on HN, I can mention that Sam Altman invested $375M of his own money in Helion, so he is indeed a true believer.
About 1/3 of that is devoted to liquid fuel transport (oil tankers), a percentage which is down from the 1970s when it was roughly half the global shipping fleet. The absolute number of tankers has increased AFAIU, but other carriers, particularly container ships, have increased in fleet size faster.
There are also bulk solid (coal) and gas / LPG (natural gas) fuel carriers, though both are smaller than the oil fleet AFAIK.
And though I focus on shipping, synthetic hydrocarbon analogue fuels would also be useful to aviation. Probably the only way that large-scale long-range commercial flight will remain viable, if it does at all.
The big 500k+ Ton (and that’s not a typo, we’re talking about 8 times heavier than the largest battleships ever built) ULCCs like Knock Nevis and Seawise Giant went to the scrapers a decade+ ago.
I'm much more sceptical on the economic viability. If it's past the edge of possibility with known materials now, there's no reason to think your reactor would suddenly last decades because you made a series kf incremental improvements from needing 10x the energy in as out to sorta do it once to 0.95x