The US Revenue Marine/Revenue Cutter Service was not a civilian organization and essentially no different from the early USCG: It was a uniformed service of the United States under the Treasury Department
And was, in that capacity, a law enforcement organization, not a military one. They, like the Federal Marshals, could and did work in tandem with the military in time of war.
OTOH, I'll again remind you that NOAA is also a uniformed service and is not, strictly speaking, a civilian agency either.
So there's one person explicitly stating that Starfleet isn't military, but others have said it is
Only two people ever implied that it was. One was David Marcus, who earlier described Admiral Kirk as a "boyscout" and later accused him of murdering the Genesis Science Team. The other was Sisko, when he describes admiral Leyton's coup de tat as a "military dictatorship."
In David's case it would be just one of a half dozen examples of him not knowing what the fuck he's talking about. In Sisko's case "military dictatorship" is a loaded term, and it's unclear exactly HOW loaded it really is. Is there, after all, a difference between a serial killer and a crazed
some Starfleet personnel identify themselves as soldiers.
ONE Starfleet officer describes himself as a soldier when talking to a bunch of sanctimonious Organians that are really pissing him off by refusing to fight for their own freedom.
What IS definitive, however, is that Starfleet has been explicitly described as a non-military organization and implicitly stated to be one far more often than the reverse. OF the counter claims, one of them is made by an angry civilian whose batting average on accurate statements is shockingly low.