... Passing this allows you to take 6 passengers on a
personal yacht, assuming it passes certain requirements. An upgrade
to a more professional license, is fairly straight forward.

The "six pack" allows you to operate an uninspected vessel near shore.
The upgrade is a bit tricky though as you need to be a US citizen and
need 720 documented sea days (typically 4 hours underway) to get a
Master 100 tons. Actual granted tonnage is often less than 100 and is
based the size of the vessels you have documented time on and the CG
may impose other limits on you license depending on your experience.
The 100 ton mate (360 sea days) includes the "opuv" and is a better
option for US citizens doing six pack trade. If you want to work
"commercial" (ie. not tourist) boats you'll likely need to get your AB
too which involves advanced fire fighting and life saving and sea time
on commercial boats. Courses for 100 ton licenses typically run three
or four weeks.

-- Tom