If the sub were on a spy mission and got caught in South Korean waters, then South Korea's actions would be justified and North Korea would be guilty of war provocation. The drowned body of an 'apparently armed North Korean agent' and his submersible propulsion unit were found on the shore in Kangwon province. The body was clad in a wetsuit and had a dual diving tank. North Korea denies that it has anything to do with the dead man. The practice of manufacturing false evidence is old as humanity.
All nations practice 'the art of deception' during war times.
Ike used a dead body planted with fake invasion plans to deceive Rommel. Yes, it is possible. Only a thorough investigation into this affair by a competent third party will bare the truth of this matter. There are well-documented cases of faked 'bodies' in Korea. Tofte set up and ran the CIA operations in Korea and China.