Unrealistic Water Crossings

[Cambyses]

naaahhh, It's something completely different to press some fishermen and merchants to get your Huge Army over the strait of messina than to mobilize a large contingent of seaworthy transport ships to traverse "open" waters(remember when crossing a short distance ships can be reused and are only borrowed for some days). An action like that would also have a large impact on local sea trade.

Plus Landbridges are also availible for Diplomats Spies etc. who would have used local transports for a few Drachma rather than waiting for a friendly fleet to come by.

Indeed, and once said area is no longer a frontier, I would imagine ferries would exist to take people across short sea straits. I wouldnt be happy if I had to recruit a fleet every time my family member needed to get from Syracuse to Rome in time for the next elections or to start his edcation. (Although Im only slightly less unhappy about doing that if he were the governor (or son of) for Africa or Hispania Inferior, but thats another debate...)

[Atilius]

my 1 impression was that Athenian and most Greek states were crewed by free men while the Romans used slaves.

That's a misconception. During most of our time period, Roman warships were crewed by proletarii - citizens who did not have enough wealth to qualify for service in the army (Polybios 6.19). The Roman navy also used vessels supplied by the allied greek city states of Magna Graecia. These vessels would hae been crewed by free greeks.