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.
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.
https://en.wikipedia.org/wiki/Nuclear_marine_propulsion