The DPRK's surface fleet mostly exist in a coastal patrol roll. They never bothered to invest heavily in it because it is neither tactically important for them nor could they hope to match the US's surface fleet. It simply isn't worth the resources it would cost them to modernize.
While not a threat to the US, china, India or Pakistan, The DPRK does have several nuclear missile submarines in development. The Sinpo-C are scheduled to come on line in 2023 - 2025, this will make the DPRK one only 4 Asian nations with nuclear capable submarines. Which is why South Korea is currently asking the US for help with developing their own nuclear powered submarines.
