exactly. diesel pigboats are great as a close in line of defense, and suck seawater for long-range offense projection...
Well diesel boats equipped with AIP(Air independent propulsion) can stay under for over 2 weeks & most diesel-electric boats coming out now-the Russian Amur,French Scorpene,German Type-212/214 & Swedish Gotlands class have AIP modules attached.Besides you don't need long endurance especially if your on a oneway trip to Guam or Hawai which is not too far from North Korea.
These would be--one way trips for the North Koreans. Who is to say they would be used on a sub? They could be placed on a peaceful trade ship in a container. When the sky is cloudy--Container opens and out comes the rockets---bye bye San Francisco, LA and San Diego. Or maybe Pearl Harbor could be the Target. The EM Pulse will burn out computers and Telephones--America will be hurt bad. And the Koreans will say "Its Terrorists--we know kothing about it."
Sink the ship and there will be no evidence of where the attack came from. Maybe I ought to sell the idea to Tom Clancy???