Mr.Midshipman Hornblower includes a fight with a Spanish galley around 1793. Wikipedia implies those ships were not used that late. Washington Post states the books are historically accurate. The author, C.S. Forester implies only a few were left at this time. Is this historically reasonable for around 1793 or did the author want to include the drama of the galley slaves in the story? The book says the Spanish galleys were well maintained, hence their existence at this date.
The term should have been "galleon," but some people at the time seem to have used the terms interchangeably, and Forester would have reflected that. Galley and galleon (and galleas) come originally from the same word, but denote different vessels. Spanish galleons/galleys of the Napoleanic period would just have been just big old sailing ships, emphasis on the big and old. They were mostly used for trading rather than war, and carrying rowers means carrying extra food and water, hence less cargo. Ever heard of a Spanish galleon escaping Caribbean pirates by rowing? A Spanish ship with sweeps would probably have been propelled by prisoners of war rather than slaves.
The Barbary pirates were still using rowed ships propelled by slaves into the early 1800's. They were rowed by actual (mostly Christian) slaves, and Europeans ofter referred to them as galleys, rather than they actual type (e. g. xebec, etc).
After you're done with Forester, I encourage you to read Patrick O'Brian's Aubrey-Maturin series (Master and Commander et al) for a much better-researched and more in-depth treatment of the subject. All of the battles are based on actual incidents, with just the names of the ships and officers changed.