The patrol boat was apparently sunk by a North Korean submarine. North Korea has several submarines, the most modern being several Soviet Whiskey-class subs from the 1950s. 
A just retribution would be the silent disappearance of those submarines, easily accomplished by a modern Virginia-class submarine designed to operate in littoral or coastal waters as well as "blue water" of the deep ocean. Use of force is authorized under our treaty to protect South Korea as well as the state war the Chinese and North Koreans refused to terminate in their negotiations to end the Korean Conflict. Just retribution would be to disappear all of North Korea's submarines, silently, no publicity, no embarrassment. It lets the Great Leader know that he cannot order the sinking of a South Korean ship with immunity, and, should he wish to escalate, he is out of his league. Everyone seems to know that except him, and his own are afraid to tell him. The US Submarine Service isn't.
