Diplomacy in potential hostile regions

[Nathan Ross]

were those slaves dedicated oarsmen or just general crewmen who had to row when the occasion required it?

There were probably few if any merchant ships driven by oars alone - most, I think, would have used oars only in calms or when entering and leaving harbour etc. They wouldn't have needed the sort of highly trained remiges that a multi-banked warship demanded either; the Synesius letter above suggests that it was easy enough to train a man to pull an oar when required. General crewmen would have done it, I expect.

Casson's Ships and Seamanship in the Ancient World contains one very brief section about the use of slaves aboard merchant ships, although rowers are not specifically mentioned in the sources he cites.