Well, it does result in enriched nuclear material floating all over the ocean, ripe for the taking by pirates or nation-states that we'd rather not have access.

Hopefully one day we'll see fusion powered container ships.

So you use the same naval threat that we already use to deter idiots from our nuclear powered vessels.

Pirates still seize large merchant ships in Asian and African waters on a monthly basis. There are only a few hundred surface vessels capable of effective anti-piracy patrols worldwide from all navies and coast guards; they can't be everywhere at once.