seaboard

rannikko The area bordering the sea; a coastline.
1948, w:Carey McWilliams (journalist)|Carey McWilliams, North from Mexico / The Spanish-Speaking People of The United States, J. B. Lippincott Company, page 25,

While De Anza was exploring the Bay of San Francisco, seeking a site for the presidio, the American colonists on the eastern seaboard, three thousand miles away, were celebrating the signing of the Declaration of Independence.