Contrary to the images that have become standard in so many novels and films, it is now believed that most Greek and Roman-era fighting ships ("galleys") were manned by free, highly trained oarsmen. While in emergency situations or under other unusual circumstances slaves might be used, this was avoided because rowing was a crucial skill for warships. Nor is there apparent evidence that criminals were condemned to the oar. Sorry, Ben-Hur.
@lauren Yeah. I hadn't given it much thought before. I guess only the military needed the manuverability and ability to travel in any direction at any time regardless of the wind.
Merchantmen can wait for favorable wind conditions.
@shuttersparks Sail was apparently the method of choice in those situations. Even then I suspect freemen were far preferred. Of course the British navy in particular essentially enslaved many men into servitude via press gangs, but that's a complicated saga unto itself.