The North Korean government has called a recent visit of the U.S. submarine Michigan to South Korea a "rehearsal" for the further deployment of nuclear submarines around the Korean peninsula.
We've been parking boats down there pretty frequently for like 40 years. I made a week long port call there on a boat [SSN 711] around 2012 - it was in their [South Korea's] news papers. This is just North K. blowing hot air. We conduct joint exercises with them faily regularly in the yellow sea.