Flights to Greece from the US typically land in Athens in the morning so you will need to go to Piraeus (the ferry port) where you can get the Ferry. When arriving at the Athens International Airport (Eleftherios Venizelos), two popular destinations are Athens and Piraeus, the ferry port where you can get a boat to the Greek Islands. Since the flights to Greece from the US typically arrive in the morning, I sometimes go directly to the ferries in Piraeus. To catch a bus to either Athens or Pireaus, walk out of the airport turn to the right and look for the city buses.
There are no ferries in Athens itself %u2013 you have to take a train to Piraeus which costs one euro and takes about 30 minutes depending on where you start in Athens.
If you're planning on going to the Cyclades, especially the popular islands like Paros, Naxos, Ios, and Santorini, there are ferries from Piraeus (Athens) every day during the tourist season”