East Sea means East China Sea./Tunghai东海 as a grobal Standard.
Korean website distorts that Europeans long years called Sea of Japan ast "East Sea" ,quotied with monority designed map. Worse case is, Korean insist it (Sea of Japan) was called East Sea, quoted from the map of "East China Sea"
Mostly,before La Perouse's voyage of discovery around Sea of Japan, some people thought Sea of Japan is just a part of East (China ) Sea so they would wrote Eastern Sea on the position of Sea of Japan.
Eastern Sea is defacto standard terms for the ocean between east of shanghai China ana Kiusiu island of ,Japan. Now Im not talking about East sea by Korean. Actually China has hegemony in East asia through the century and every knowledge and information was reported and recorded in China. They call it Eastern Sea (Tung-hai东海 ) , the ocean between East of Shanghai area to Jeju island and Kyusyuu island, incruding Suyan-Rock苏岩礁. Most of European Maps referemced the name of Eastern Sea or Oriental Sea, Mer du O'st, Ostrichmeer are referenced from those Chinese map.
China was well-known to European and their geography knowledge is quoted from Chinese geographic infomations . China produced enormous number of Geographic magazine and Maps. I just show some maps which describes Chia Eastern Sea/Tung-hai , which is the original description of ,Mer du Cin, Mer du Orientale/Oriental Sea/Mer Bleue/Ostchinesche Meer) described between China East area and Korean Peninsula.
historical Chinese map "Eastern Sea"(Tung-hai)
1135.歴代地理指掌圖 古今華夷区域総要圖 （China)
東海 East （China) Sea between China continent and Korean penunsula.
1351.Yu-kung's 書集伝音釈 禹貢九州及今州郡之圖 （China)
東海 East （China) Sea between China continent and Korean penunsula.
By the way, Korean northeast history foundation insist Korea use East Sea from 13th century ,quoted from this map. But we know it is appear that Korean claim is distorture and that East Sea is "East China Sea".
1584.人子須知地理心学 中国三大幹龍総攬之圖 （China)
東海 East （China) Sea between China continent and Korean penunsula.
1613.図書篇 四海華夷総圖 （China)
東海 East （China) Sea between China continent and Korean penunsula.
1640..四書引蒙翼経圖解 天下三大幹附歴代帝都九州二十八宿分野総圖 （China)
東海 East （China) Sea Just a East of China continent.
Sea of Japan (日本海）
東海 East （China） Sea located on between Shanghai and Japan
地理全志 is "All-Geography book" written in China which introduce's european Geography study. They already use the name of "Sea of Japan日本海”
http://www.wul.waseda.ac.jp/kotenseki/html/ru02/ru02_00168/index.html
Voluome 1 History of Asia
http://archive.wul.waseda.ac.jp/kosho/ru02/ru02_00168/ru02_00168_0001/ru02_00168_0001.html
Related likn (Wikipedia 东海）