They're using 50's-era naval ships and boats. Nothing they have today can evade modern radar or tracking systems, and head-on their entire fleet could be destroyed easily. They could do damage in a very short window, and that's the concern: that they'll just fire everything they've got at anyone nearby before the dozens of ships and hundreds of weapons aimed at their military infrastructure can do anything. Unfortunately there's no proven method to defeat artillery in-flight, but South Korea is practically riddled with shelters and they practice for this. They've got planes, tanks, ships, and subs, but they also have a peasant military with uneducated, likely untrained force.