DHS considers all open water boundaries to be international waters therefore the shores are within their search authority and jurisdiction. The US Border Patrol therefore can search any person or vehicle within 100 miles of the boundary without a warrant or court order.
If I come by boat from Canada to Chicago. Chicago is an interntional port with a US Customs Office and a US Border Patrol team on duty 24/7 waiting to search me, my luggage and the boat if they wish.
I suppose they could press the point on within 100 miles of an international airport if they really wanted to. Or within 100 miles of the Mississippi and other larger rivers.