Reminder that galley slaves were an early modern phenomenon that did not exist in middle ages and was very rare in antiquity!
In middle ages being a galley rower was a very honorable position that required great skill!
I explained this in this thread.

Galley slaves only began to be used on mass scale in 16th century due to growing power of centralized states and big naval conflicts between Ottomans and Christians which depleted the pool of talented and skilled oarsmen while increasing amount of captives on both sides.

This is why inferior methods of rowing were introduced that replaced the skilled medieval methods that required each man having his own oar.
The galley slaves were neither skilled not motivated so they just gave them one oar per bench and forced them to row.

But in the mind of Western people they associate galley rowers with slaves and convicts, the association that originated only in 16th century.
Also because of movies like Ben Hur, even though Romans did not use galley slaves. They also had skilled free men for rowers.