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.
The seaboard is the part of a country that is next to the sea; used especially of the coasts of North America. the Eastern seaboard of the USA. = coast. the part of a country that is near the sea eastern/western/Pacific etc seaboard