I've written on this before. A key thing to note is that there was pretty much no such thing as a galley slave in the ancient world. Rowers were free men, or if there was a manpower shortage they would be freed before their service (except in exactly one instance I know of, in which Scipio enslaved a captured city and promised them freedom afterward). This is a key feature of navies in the ancient world, and one of the major reasons for the general lack of large standing fleets aside from the Athenians, who then had to deal with the political implications of using poor citizens as the primary means of the defense of the city