The USCG Master's License is the highest level license that you can acquire. This license allows you to operate inspected vessels (7 passengers or more – like a water taxi or dive boat) or uninspected vessels.
The USCG does not require that you get a Six-Pack License before you get your Masters, so if you qualify, you can skip the Six-Pack and go straight to your Masters!
A Master license is issued for operation on the Great Lakes, Inland and Near Coast waterways. This license is issued in 3 versions: 25; 50; or a 100 Ton License. Both the amount of sea service time and the size vessels you have been on will influence the license you are eligible for.
Here is a link to a short video that will give you additional information on that level license: http://goto.marinerslearningsystem.com/25-50-100-Ton_Master_License_Video.html