Mass. Gen. Laws ch. 34 § 1
The seaward boundary of counties bordering on the open sea shall coincide with the marine boundary of the commonwealth. The boundary lines in tide water between adjacent coastal counties shall coincide with and are hereby established to be the boundary lines in tide water between the adjoining coastal municipalities of said counties, as confirmed and established by section one of chapter forty-two. Counties separated by waters within the jurisdiction of the commonwealth shall exercise a concurrent jurisdiction over such waters. Each county shall be a body politic and corporate for the purposes of suit, of buying and holding, for county uses, personal estate and land lying therein, and of contracting and doing other necessary acts relative to its property and affairs.
Mass. Gen. Laws ch. 34, § 1