The basic licence you refer to "six pack" no longer is available from the USCG. It is now called OUPV which stands for operator of un-inspected private vessel. Go for the masters, it is only one more module on the test. You will need to document your sea service on your application. The service time can be from the operation of your own vessel; one does not have to have a job "on the water". I hope you kept an accurate log; it makes the application process much easier.