The Aktina travel agency is located on the arrivals floor of the Athens International Airport “Eleftherios Venizelos”.
You can book all greek ferry tickets here including high speed Hellenic Seaways, SeaJet, and Blue Star ferries. You can book ferries from island to island as well (not only ferries that originate in Athens). You can also book flights on Olympic and Aegean airlines.
I believe the agency is open 24 hours a day but I would not be surprised if it had reduced hours during low season.
The Aktina website is here – and like many Greek websites it is completely useless.
Aktina phone: +30 210 35 30 160
Aktina email: firstname.lastname@example.org