I was reading Roger Crowley's history of Venice, and it mentioned how earlier in the middle-ages, galley-oarsmen were proud, skilled workers, with reasonably good social status, like artisans or the like. But by the 16th century, it was prisoners, slaves, and similar low-status people who rowed, what happened to cause this transition?
A contributing factor was that the 16th century was the start the Price Revolution, as economists put it: a time of rising wages and rising prices all around. What caused it is still debated. (The influx of New World silver and gold was probably a factor.) With rising wages, free galleymen--who were very skilled workers--would become unaffordable, and slaves and convicts would be a cheaper alternative.
There is a twist, though. In The Tactics of the Battle of Lepanto Clarified: The Impact of Social, Economic, and Political Factors on Sixteenth Century Galley Warfare (a copy on the author, John F. Guilmartin's, personal Angelfire site is here), Guilmartin states that Venetian war galleys still relied on free rowers, in marked contrast to the servile rowers that Spain and the Holy League's galleys relied on. Free rowers are generally preferable at a purely tactical level, since they're more skilled and can execute far more maneuvers than the slave rowers, and they don't require fighting men to be tied down as guards. (Just the opposite, in fact: they were armed and were used as boarding infantry.) And because a) wages and prices were lower in the Eastern Med than in the west, and b) Venice, as a mercantile republic, had more wealth to spend than the poorer, manpower-heavy Spanish Empire, Venice could afford the salaried rowers.
That's just at Lepanto, though, and chasing down the footnote gives us this:
[37]Both Venice and the Ottomans retained free ciurmi on their capitanas and patronas after they were forced by economic pressures to adopt largely servile ciurmi [rowers] on ordinary galleys. The first regularly sanctioned use of convicts as oarsmen on Venetian galleys did not occur until 1549. re Tenenti, Cristoforo da Canal, pp. 83, 85. See Tenenti, Piracy and the Decline of Venice (Berkeley, 1967), pp. 124-25, for Cristoforo da Canal's backhanded testimony to the tactical effectiveness of free oarsmen c. 1587 (he was mainly concerned with their higher cost).
I think that largely sums it up: salaried rowers were expensive and getting more expensive, and in day-to-day economic cruising enslaved rowers would work just as well. The elite salaried rowers could be saved for warfare.
(e: formatting)