Why doesn't South Korea just torpedo the Nork sub when it is submerged? Plausible denial.

And 99% start the shooting war a prosperous S. Korea has been avoiding like the plague for 65 years. While a wealthy successful south will swallow its pride, insults and even the occasional corvette, i think you'll find an incredibly bellicose North would lurve the opportunity to go toe to toe. They've nothing left to lose, and no allies left to speak of.