How is the term "Coast" defined for FAA regulatory purposes? 14 CFR 91.1 defines the applicability of part 91. This paragraph defines two areas of jurisdiction where certain parts of part 91 apply:

within the United States, including the waters within 3 nautical miles of the U.S. coast
the airspace overlying the waters between 3 and 12 nautical miles from the coast of the United States

By exclusion the third area is the airspace overlying waters beyond 12 nautical miles from the coast of the Unites States. Part 91 therefore does not apply in this area. The FAA explicitly states this in letters of interpretation, for example, in Di Marco (2008).
The term &quot;coast&quot; appears to be undefined in any readily available FAA resources.
How are we to understand this term as applied to the scope of applicability of §91? For example, would a shore of one of the Great Lakes—Lake Huron for example—be considered a &quot;coast&quot; for this purpose? To apply the question more specifically, would a pilot be required to comply with the 250 knot rule of §91.117 if below 10,000 ft while 15 miles off the US shore over Lake Huron.
Coastline means the territorial sea baseline. In the language of international treaties like UNCLOS, the territorial sea doesn't include internal waters, regardless of their size. NOAA confirms this, notes however also, that not all US court decisions follow that view. NOAA also provides a map to view the US territorial waters &amp; exclusive economical zone, which shows that the great lakes are neither.