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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