Well, the definition in Black's Law Dictionary, which is usually a reliable source in the US, says:
"MASTER OF A SHIP. In maritime law, the commander of a merchant vessel, who has the chief charge of her government and navigation and the command of the crew, as well as the general care and control of the vessel and cargo, as the representative and confidential agent of the owner. He is also commonly called the "captain". "
So if you were put in charge of the vessel by the owner when he was not present...that would seem to make you the "master" for those times.