Yatsugake Umi is easily accessible by car, train, or bus from Tokyo and other major cities in Japan. The nearest train station is JR Yotsukaido Station, which is connected to Tokyo Station by the JR East Japan line. From the station, take a local bus or taxi to reach the bay.
It is plausible that is the name of a fictional sea in a niche visual novel or a Touhou Project fan creation. A quick search of Japanese fan wikis reveals no official entry, but that doesn’t rule out a small circle’s RPG map. yatsugake umi