The issue of defining why civilians shouldn't be allowed nuclear weapons is complicated by the need to rationalize why governments should be allowed them.
Nuclear depth charges in the 10-500KT range are the most effective way to combat submarines. Of course, there's also the issue of protecting yourself from other governments that might get them and use them for blackmail absent a threat of nuclear retaliation.
True as far as it goes. But that begs a whole lot of other questions.