Section 1. The seaward boundary of cities and towns bordering on the open sea shall coincide with the marine boundary of the commonwealth. The boundary lines in tide water between adjacent coastal municipalities, as located and defined by the board of harbor and land commissioners under chapter one hundred and ninety-six of the acts of eighteen hundred and eighty-one, except in so far as the boundary lines so located and defined purported to vary boundary lines in tide water between municipalities theretofore established by the general court, together with such boundary lines theretofore so established, as such boundary lines have been subsequently changed by the general court or defined by decrees of the land court, with such additions to or subtractions from the length thereof as have been made by natural changes in the shore line of the commonwealth, are hereby confirmed and established as the legal boundary lines in tide water between said adjacent municipalities; provided, that such boundary lines shall hereafter be changed from time to time to conform to any change in said shore line.