You can fly to several different airports in Greece.
The possibilities are following:
Athens airport: if you start your cruise in Athens or Piraeus this is, of course, the best airport to start your cruise. You can cruise from Athens to Ionian islands, Cyclades, Sporades and Saronic islands. And transfer from airport to gulet is fast and interesting. The only problem can be heavy crowds.
Airports which you can find on several islands: you can also fly directly to Mykonos, Rhodes, Kos, Zakynthos, Santorini or some other of the islands. A huge number of tour operators fly there on regular basis.
Turkish airport: you can also fly to Bodrum airport or Dalaman airport if you decide to cruise Dodecanese islands, start your cruise in Turkey and then go to Greece with a gulet.