The water is a heavily disputed area between South Korea and Japan.
Japan has been trying to claim this sea as its own against historical records.
But, what is for sure is East Sea has been the South Korean territory for so many years.
The name "the East Sea" appears in the History of the Three Kingdoms, one of the Korean ancient history books, in around 59B.C., while Japan's record of calling it "The Sea of Japan" appeared much later.
Not only that, in practice, the East Sea is under South Korea's control and the Dok islet in the East Sea is populated only by South Koreans.