tyboka sea
An Isthmus
The Red Sea separates North Africa from Southwest Asia
The Atlantic Ocean
hollywood
The body of water that separates the three main southern islands from the rest of Japan is the East China Sea.
The Korean Strait
The Artic Ocean
The Yellow Sea and East China Sea
Black Sea
the red sea
If you are referring to seas, the Japan sea and the Pacific ocean are on either side of Japan. If you mean the body of water between Hokkaido and Honshu it is called the Tsugaru Strait.