You can easily get cash from ATM's in Turkey and they tend to take credit cards. Can't comment on the Greece situation because it changes daily, sorry. But Kos is having a problem at the moment with the huge influx of people fleeing Syria and trying to get into the EU. BBC news reports people landing on the beaches nightly and sleeping everywhere. Kos isn't able to handle all the new arrivals.

We spent 2 years sailing in Turkey/Greece we liked it so much. But do watch out for the Meltimi