To get from working on the deck of a towboat to working in the wheelhouse, the primary requirement is experience, which is documented by sea-time.  In turn where that experience is acquired -- Oceans, Near Coastal, Great Lakes, Inland waters, Western Rivers or "limited local areas" -- determines which license is available to the prospective officer.

First off, we need to define each of those areas of operation.

• Oceans means the waters seaward of the Boundary Lines as described in 46 CFR Part 7.  For the purposes of establishing sea service credit, the waters of the Inside Passage between Puget Sound and Cape Spencer, Alaska, are not considered Oceans.
• Near Coastal means waters not more than 200 miles offshore.
• Great Lakes means the Great Lakes and their connecting and tributary waters including the Calumet River as far as the Thomas J. O'Brien lock and Controlling Works (between mile 326 and 327), the Chicago River as far as the east side of the Ashland Avenue Bridge (between mile 321 and 322), and the St. Lawrence River as far east as the lower exit of St. Lambert lock.
• Inland Waters means the navigable waters of the United States shoreward of the Boundary Lines as described in 46 CFR Part 7, excluding the Great Lakes and, for towing vessels, excluding the Western Rivers.  For establishing credit for sea service, the waters of the Inside Passage between Puget Sound and Cape Spencer, Alaska, are Inland Waters.
• Western Rivers means the Mississippi River, its tributaries, South Pass, and Southwest pass, to the navigational demarcation lines dividing the high seas from harbors, rivers and other inland waters of the United States, and the Port Allen-Morgan City Alternate Route, and that part of the Atchafalaya River above its junction with the Port Allen-Morgan City Alternate Route including the Old River and the Red River, and those waters specified in 33 CFR 89.25.
• "Limited local area" means a defined limited geographic area designated by the local OCMI.  For example, a license might be issued as "Master of Towing Vessels (Limited) Upon the Western Rivers on the Ohio River from Mile 743.0 to Mile 747.0."  The holder of this license could not legally operate anywhere other than this 4-mile stretch of the Ohio River.

Now to define terms used in calculating sea-time.

• Day means eight hours of watchstanding or day-working not to include overtime.  On vessels where a 12-hour working day is authorized and practiced, such as on a six-on, six-off watch schedule, each work day may be creditable as 1.5 days of service.  On vessels of less than 100 gross tons, a day is considered as eight hours unless the Officer in Charge, Marine Inspection determines that the vessel's operating schedule makes this criteria inappropriate.  In no case will this period be less than four hours.  Most, but not all, deck time on towing vessels is time-and-a-half -- a "day" is calculated as 1.5 days.
• Year means 360 days for the purpose of complying with the service requirements for licensing.