Perhaps I see things a little differently. I see Captain more of a universal title. I grew up near the water and the majority of work choices growing up was either work on a mud boat or on a shrimp boat. As a teen I pondered the question what makes one a Captain or a Master of the vessel or even a skipper? What made sense to me all of those years ago was this…
If you work on a charter fishing boat or tour boat under 100GT then you are a Skipper.
If you work on a trawler or fishing boat then you are a Captain. (colloquial title not necessarily licensed)
If you work on a vessel and there is more than one licensed mariner working on the bridge then you have a Master and a Mate.
All of the above may be referred to as a Captain because they are in charge of the vessel at a particular time. However, this does not count if you only have a Mates license for that vessel. Only licensed Masters for the particular tonnage they are on can use the Captain title.
When I worked foreign there were seven of us on the bridge. I was the Master of the vessel. I had two licensed Masters and two licensed Mates (US flag) and two licensed Mates for the country we were working in.
I worked 0600-1800 as Master of the vessel. (Since I have no one working an opposite watch of myself I had no chief mate.) The two licensed Masters under me worked 12-12. Each one had two licensed Mates under him one American and one foreign national.
Each one of the licensed Masters under me was OICNW of his watch and was refereed to as Captain of the watch. So in terms of the vessel there was one Master and two Captains and four Mates.
Ranking order would be Master of the vessel, then Captain then Mate or Chief Mate, Second Mate and Third mate if applicable.