Shipboard troops or troops serving on a vessel are called Marines
Macos are marines (low rent ones)
Not necessarily. Until the 90s the Chinese Navy was considered part of the Army. The largest amphibious assault in world history was conducted by Allied troops including the US Army, which- at the time- had it's own boats and landing craft. The US Marines were busy elsewhere. The US participation in the invasion of Japan would have consisted of US Army divisions.
Major Hayes referred to training at an Army post, which would subtly identify them as soldiers. In addition, all of the named characters are referred to by ranks used by armies- they don't have any Lance Corporals or Gunnery Sergeants, ranks unique to marines.
Given that many marines have rich histories it is likely they would have also had distinctive uniform items like the EGA (US Marines) or the green beret (Royal Marines). The MACO insignia is more like an army unit patch- and positioned that way on the uniform.
Originally Posted by sollvax
The Royal marines were the first fully uniformed disciplined marine force
The USMC is not the only kind of Marine
True, the Royal Marines were the first military units to be identified as Marines- or originally a "Maritime Regiment Afoot." If you mean that not all Marines have the global reach and diverse capabilities of the US Marine Corps, but all are amphibious infantry, that's true.
Is still think MACOs are soldiers. But, more to the point, the OP suggested having the option of enlisted ranks and MACO ranks. I would agree, at least for our BOFFs.