Great question, here is what I have found:
The term "Inland Waters" refers to
those waters shoreward of the boundary line.
Title 46: Shipping
PART 7—BOUNDARY LINES
Gulf Coast
§ 7.105 Marquesas Keys, FL to Rio Grande, TX.A line drawn from Marquesas Keys, Florida at approximate position latitude 24°47.5' N, longitude 82°11.2' W; along the 12-mile line which marks the seaward limits of the territorial sea (as defined in 33 CFR 2.22(a)(1)) to Rio Grande, Texas at approximate position latitude 25°58.6' N, longitude 96°55.5' W.[USCG-2001–9044, 68 FR 42602, July 18, 2003]
It would seem as though the "boundary line" for the Gulf of Mexico is 12 miles out by CFR Title 46, part 7, 7.105. That would allow an inland licensed captain to operate up to the territorial sea limits (12 miles).
I am still not 100% on this one and welcome input from anyone with a solid understanding of the matter.
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