The Missouri River and its mainstem impoundments, Lake Oahe and Lake Sharpe, are South Dakota's navigable water, and the river is within the 33 CFR 83.03(n) Western Rivers definition. Whether a mainstem reservoir behind Oahe Dam carries the same provisions is an OCMI determination rather than an inference, and this page does not make it. No Part 80 demarcation line exists in South Dakota, so everything here is inland water under 83.03(q) and the Inland Rules apply. For candidates whose real sea service was earned on the coasts or the Great Lakes, the useful discipline is a record that separates each water by name, dates and vessel — REC St. Louis will make the route determination from that, not from the applicant's state.
33 CFR Part 80 draws no demarcation line in South Dakota. Its 106 sections cover the Atlantic, Gulf and Pacific coasts, Puerto Rico and the Virgin Islands, the Pacific islands and Alaska; a state with no coast has no section, which is itself the answer.
Western Rivers (the Missouri River) are defined at 33 CFR 83.03(n) and carry Inland Rules provisions the rest of inland water does not: 83.09(a)(ii), 83.14(d), 83.15(b) and the towing-light exception at 83.24(j).
This decides which rulebook you steer by, not where your credential may operate: the route limit is the 46 CFR Part 7 Boundary Line, and the two lines are not drawn in the same place. How the two lines differ · International Rules · Inland Rules
33 CFR Part 80 and the Part 83 definitions above read from eCFR on 12 Aug 2026. Any specific water is a determination for the cognizant Coast Guard Officer in Charge, Marine Inspection.