"Who is the captain?"
Translation:Ποιος είναι ο καπετάνιος;
No, καπετάνιος refers to all ships that have a crew, military or not. A correction. Did you know that the title captain is a Medieval Greek word? From the word "κατεπάνω" a title which had been used in the Byzantine Empire. More info here: https://en.wikipedia.org/wiki/Katepano. And is for sure of Latin origin as you can see in the link (caput, Gen. capitis etc) from which come many other words as capital, capuccino monk and then ... coffee etc. :) Amazing.
Ευχαριστώ! To complete the meaning of καπετάνιος: This term concerned also the commander of the "klephtes and armatoloi" during the Ottoman period and the Greek Revolution. See: https://en.wikipedia.org/wiki/Klepht Next to it and in accordance with, it was used for the chiefs of guerilla war during the WW II period. See, about Greek Resistance here: https://en.wikipedia.org/wiki/Greek_Resistance .
In both cases it concerned the leaders of war. In the same time it was using the title for the Navy as always, the captain of the ship. The corresponding official Army rank title is "λοχαγός", but there is not an equivalence in authority with "καπετάνιος", λοχαγός is the leader of λόχος, an Ancient Greek military unit, that was not the same as today. Λόχος is the military unit of about 100-300 men in the modern Army. In English it could be translated as "company". About "λογαγός", see: https://glosbe.com/el/en/%CE%BB%CE%BF%CF%87%CE%B1%CE%B3%CF%8C%CF%82 . Captain could be the leader of larger or smaller units in the guerilla war in the Ottoman Period and the Greek Resistance. It was a title of honor, even he had the power of life or death on his men.
The English word "captain" means two different things in Greek! A ship captain is indeed καπετάνιος, BUT an airplane captain is a κυβερνήτης! The question should have specified! And as stated in the comments below, it could also be λοχαγός ή πλοίαρχος. The correct category should have been stated in parentheses after the question. . . (of a ship, or of an airplane)