There are a book titled "Painful history of Korea" by Park Eun-sik who is independent movemennt activist. Even Korean independentist use the name of Sea of Japan and he distinguish Korean coast and the deep sea -Sea of Japan.
According to the articles by Prof Shimojo and Sen.Sindo, Korean recognizes east coastal water and deep sea are different, east coast and the Sea of Japan witten in Painful History of Korea.
「韓国は亜細亜東南の突出に在る半島国なり。その境界は東、滄海に濱(沿って)日本海を隔て、西は黄海に臨んで中国の山東江蘇二省に対す」
"Korea is the peninsula country at the southeast of Asia. There are east boundary of Blue Sea which separate to the Sea of Japan, west one of Yellow Sea,to the China's two proince-Shandong and Jiangsu."
reference;
実事求是第35回 竹島問題の封印策としての「東海」呼称について
http://www.pref.shimane.lg.jp/soumu/web-takeshima/takeshima04/takeshima04-2/takeshima05-q.html
A Study of the Naming Issue of Japan Sea Yoshitaka Shindo Member of the House of Representatives National Diet of Japan
http://www.shindo.gr.jp/%E8%B3%87%E6%96%99%E4%B8%80%E5%BC%8F%EF%BC%88%E7%B1%B3%E5%9B%BD%E3%81%82%E3%81%A6%EF%BC%89.pdf
If someone have a screenshot of Painful history of Korea, pls advice. thank you.
There are another korean records textbook written by Korean before the annexation, which distinguish east coast and the deep sea of Japan Sea.
1909.1910.
(新訂)中等萬國新地志 金鴻卿 編纂 廣學書館
http://seaofjapan-noeastsea.blogspot.jp/2009/07/1907.html
第二章 亜細亜 (Second chapter;Asia)
第一節 総論 (First Section: general)
沿海 P52 (Coastal sea)
日本海、渤海、黄海、東海、朝鮮海(別称蒼海)、支那海、逞羅湾
..............................................................................
右諸海中、北氷洋 舟編 不通●●●●海●●●日本海.....
Japan Sea, Bol-hae,Yellow Sea, East Sea(*East China Sea?), Korean Sea(another name;Blue Sea) and China Sea (south china Sea*), Gulf of Siam,
...........................................................................
..............................................................................
右諸海中、北氷洋 舟編 不通●●●●海●●●日本海.....
Japan Sea, Bol-hae,Yellow Sea, East Sea(*East China Sea?), Korean Sea(another name;Blue Sea) and China Sea (south china Sea*), Gulf of Siam,
...........................................................................
in those sea at the north Pacific.................Sea of Japan.
reference;
実事求是第35回 竹島問題の封印策としての「東海」呼称について
http://www.pref.shimane.lg.jp/soumu/web-takeshima/takeshima04/takeshima04-2/takeshima05-q.html
A Study of the Naming Issue of Japan Sea Yoshitaka Shindo Member of the House of Representatives National Diet of Japan
http://www.shindo.gr.jp/%E8%B3%87%E6%96%99%E4%B8%80%E5%BC%8F%EF%BC%88%E7%B1%B3%E5%9B%BD%E3%81%82%E3%81%A6%EF%BC%89.pdf