C.E. Evans wrote:
I think Starfleet's security division does have personnel that are Starfleet Marines in everything but name.
I think that fits the on-screen evidence best. Starfleet's shipboard security teams seem a good match for the early incarnations of US or British Marines. Just as the US Navy began combining shipboard detachments into a larger Fleet Marine Force, which in turn evolved into something bigger and much more extensive, I can see Starfleet's shipboard security being the initial basis of something that became (or could become, if needed) a larger and semi-independent "ground" force.
As for terminology, as an all-rolled-into-one "combined service" Starfleet may intentionally avoid terms traditionally tied to one environment like "army," "navy" or "marines." The titles of ranks don't really mean much in the long run; while Britain and the US put army-type units aboard ship as marines, other nations worked the other way and organized sailors into marine/naval infantry forces with naval-style titles, insignia and dress uniforms. And for 70 years the US Navy has had Seabee units with naval-type ranks running land-type units -- regiments, battalions companies and so on -- and it has worked fine.