The Roman fleet did not use slaves for service at sea. The boatmen, sailors and naval sailors were ordinary auxiliares who in exchange for a good 26-year military service could expect them to be granted Roman citizenship.
Gaius Octavius during the war with Sextus Pompey (43-36 BCE) decided, by way of exception, to recruit 20.000 slaves on board of ships, which he enlisted in the oars. First, however, he gave them freedom.
Appian of Alexandria, Roman History
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