The boundary line is defined in 46 CFR part 7. Here's a link
that gives an overview, and a link to the CFR text.
To quote from that page: "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 coincidentally overlap at places along the coastline."
It sort of unfair to New England
boaters, who in some places have to go a lot farther out to get a sea-day outside the line, but these rules weren't really set up for recreational boaters anyway.