They would be lucky to make it to Japan, Hwanghae is on the west coast, north of Incheon.
How many torpedoes do these boats carry? they need not go far to cause a great deal of problems for a trading nation like South Korea. If they can launch even primitive rockets they could menace a wide range of targets—what about our base in Okinawa? Pusan Harbor? Just going missing is a threat. What ever they do—it will happen in the next 48 hours.
These Yono Class midget submarines have a range of about 50 nautical miles submerged and about 550 nautical miles on the surface.
The larger Sang-O-II submarines have a submerged range of 1,500 nautical miles or more if the Norks have increased their range with some later modifications.