I wonder, is it not still technically legal anyways? (ignoring NFA restrictions for the moment...) What is the exact law here? A company can have armed security at their port terminal, and at the docks. Can they not have an armed boat?

On the other hand there's a big difference between "it's ok to use lethal force in self defense" and "you can shoot at dangerous looking ships at your discretion and maybe proactively sink them if they keep harassing you."