The Boundary Line marks the dividing point between internal and offshore waters for several legal purposes, including load line regulations.
The Boundary Line is sometimes confused with the Demarcation Line, which is the dividing point between domestic rules-of-the-road (Inland Navigating Rules) and the international rules-of-the-road (Convention on the International Regulations for Preventing Collision at Sea, or COLREGS). The Boundary Line and Demarcation Line are different lines for different purposes, although they might coincidently overlap at places along the coastline.
As a rule, on the Atlantic, Pacific, and Great Lakes coasts, the Boundary Line generally follows the high water shoreline, extended across the entrances to small bays, inlets, harbors, rivers, the ends of breakwaters or jetties, etc. In most cases, this means that as soon as an outbound vessel crosses seaward of the entrance, it has crossed outside the Boundary Line.
There are some significant exceptions to the above general rule, however:
In the Gulf of Mexico between the Marquesas Keys, FL, and the Rio Grande river mouth, TX, the Boundary Line is located 12 nautical miles offshore. This creates a coastwise marine corridor where non-load line vessels may operate;
In New England waters, the Boundary Line follows a series of lights and offshore buoys from West Quoddy Head Light (at the U.S./Canadian border) to Race Point Light (at the tip of Cape Cod). Like the Gulf of Mexico, this also creates a coastwise corridor inside of which all vessels may operate without a load line. However, unlike the GoM Boundary Line (which is a constant 12 NM from the coast), the New England Boundary Line does not follow the coastline and therefore is an irregular distance offshore.
For this reason, operators of non-load line vessels--including fishing vessels that are more than 79 feet in length and that are built on/after 1 July 2013--should especially familiarize themselves with the Boundary Line in New England waters (refer to 46 CFR Parts 7.10 and 7.15). Taking such a vessel outside the Boundary Line constitutes a load line violation;
And at other points along the U.S. coastlines, there may also be some local gerrymandering of the Boundary Line from the mouths of inlets or jetties out to the sea buoy and back.
Therefore, it is essential to consult 46 CFR Part 7, in conjunction with the appropriate nautical chart, to determine the specific location of the Boundary Line for any particular location.
Contact the Naval Architecture Division:
U.S. Coast Guard Headquarters (CG-ENG-2)
2100 Second Street S.W. -- Mail stop 7126
Washington, D.C. 20593-7126