Actually, according to a resource someone helpfully posted, Chia Head has 88 submarines (of some description or other) of his own. If half of them are actually seaworthy and duty ready, that would be enough to create a problem in itself if Chia Head decided to try to close Korean waters. The Seoul government has about half that many, but more surface ships.
In todays modern warfare techniques, I don't believe the NK military, including their navy would stand a heart beat of a chance to win a conflict. The only thing that saved their butts in the Korean War, was a massive, large Chinese army on the other side of the Yalu River. Otherwise, we defeated the NK army big time, and if we would have had the balls, would have chased, and defeated the Chinese army too.