We don't have synonyms for east sea yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are at sea, east asia and salt sea.
(n) :
A number of other seas, called locally “East Seas" in various languages; these include the Baltic Sea, Sea of Japan, East China Sea and South China Sea.
(n) :
A number of other seas, called locally “East Seas" in various languages; these include the Baltic Sea, Sea of Japan, East China Sea and South China Sea.