We are a commune of inquiring, skeptical, politically centrist, capitalist, anglophile, traditionalist New England Yankee humans, humanoids, and animals with many interests beyond and above politics. Each of us has had a high-school education (or GED), but all had ADD so didn't pay attention very well, especially the dogs. Each one of us does "try my best to be just like I am," and none of us enjoys working for others, including for Maggie, from whom we receive neither a nickel nor a dime. Freedom from nags, cranks, government, do-gooders, control-freaks and idiots is all that we ask for.
Captain is a rank; Master is a position.
Traditionally, merchant ships are divided into 2 departments: Deck (i.e. navigation) and Engineering (power). The Chief Officer is in charge of the first, the Chief Engineer of the second. Over both is the Master.
Someone can hold the rank of Captain and be the Chief Officer on a ship.
In everyday speech, though, the head of the ship is often called the Captain, and it seems somewhat pedantic to insist on the distinction.
Master, of course, comes from the English where it does not have the connotations of plantation slavery that it does in the US.
The term is still used, but here "Master" is an adjective. It means "super-duper," i.e., the person is, by skill and experience, well above the level of an "Able-Bodied Seaman."
BTW, there are separate US unions for the officer-level people and those for regular seamen. (Although the preferred, gender-neutral term these days is "mariners.") Seaman have the Seafarers International Union, and officers have Masters, Mates & Pilots.
It is my understanding that - in the British merchant marine - a sailor working his way up the ladder always found himself in a rank one level below the exam last passed. In other words, a sailor who had passed the exam for second mate was still rated as third mate. That has seemed to me to be a good system, as the sailor would have the book knowledge of the senior rank to assist him in gaining experience while he served at a lower rank.