Bit of a nit, but galley slaves did not exist except from the 16th to 18th centuries.
In ancient times, let’s assume you had 300 free rowers and 100 sailors and marines on board. In a boarding action you had 400 fighters. If your rowers were slaves and the other guy’s were free, your fighters would be outnumbered 4:1, not to mention the obvious danger of your own rowers turning on you.
During the 16th century cannon became the primary weapon system, and the rowers were demoted to mere propulsion. This was when galley slaves came in. If they’re not going to fight anyway, you might as well use your criminals for something useful.
This wreck is supposedly way before then, back when the Sahara was navigable.