Most bulk carriers are a string of boxes, again with a double hull.
The three most vulnerable types are tankers, ferries and passenger ships. Tankers because of the cargo. Ferries and cruise liners are the worst, since they are one large open space, not sectioned off into watertight spaces, since hatches annoy civilians. Even with a double hull, they are top heavy. Cruise lines don't like ships that roll. Ferries have a shallow draft. Seasick passengers don't have fun. They use gyroscopes and fins to retain stability. Cruise line crews, based on what I've seen, don't respond well to emergencies.
The DPRK navy has only two classes of blue water boats. 4 ex-USSR Whiskey class boats, and 22 Romeo class, some Chinese built, the rest kit built by the Norks. They can reach our west coast on the surface, which means they would probably be picked up by our systems and 'prosecuted'. The question would be, "Why bother?"
Even running with a snorkel, diesels are loud, at least as subs go. The Romeo class is 140db, which is freight train loud. Electric motors are quiet, but range limited by the batteries altho range is probably 10 times or more what it was during WW2. (That's classified.)
Boiled down, the big boats will be murdered, the midgets might kill some local ferries or some bulk cargo ships. Either way, I doubt they will have a port to come home to.
I was envisioning some of their subs operating in tandem with freighters wherein they were refueled by the freighter and used them as cover.
Kido Butai tried to avoid the main sealanes when they sailed in November 1941 and were fairly successful at avoiding detection. Hard to know how competent the Nork Navy leaders are at strategic thinking. They will lose a big war but could wreak a lot of damage before defeat.
You know, it would seem that the speed of modern vessels is likely near double the ordinary 40s steamer. Make them a little harder to attack.
The Yellow Sea averages about 150 feet deep so perhaps they just use their smaller subs there.