The SLC states "To gain an SLC, the candidate must hold a national government boating license (NASBLA State Boater License for USA or PCOC for Canada), …”
NASBLA is the national governing body for the USA for boating licenses as appointed by the Federal Government. Each State and territory in the United States subscribes to issuing a NASBLA approved license.
Many states do NOT REQUIRE you to have a NASBLA license if you were born before a certain year - e.g. 1995 in many cases. But in that State, if you were born after the State's cut off year, you ARE REQUIRED to have a NASBLA license.
Many other States DO REQUIRE EVER BOATER to hold a NASBLA State Boater License.
If you hold a license in one state, by reciprocity, you can legally boat in another state (where required).
Sometimes the State boat License is referred to as a State Boater Card. None the less, it is issued via NASBLA
For the USA then, it is your NASBLA state boating license that is the National Government License. It is for this reason that the SLC requires you to hold a State boater license approved by NASBLA.
To gain a NASBLA State boater license is free, simply go to www.boatus.org/free and select your state. If it is not listed there you can get any other State's license. Take the online course - grab a very large cup of coffee before hand.