I've been in Greece just once, in a well planned trip that, in a week, took us to three of the most beautiful islands. staring and ending in Athens.
The chosen islands were Santorini (the most interesting), Mykonos and Samos.
The connections were made by plane, except from Santorini to Mykonos where we had to pick the ferry for a boring 9 hours trip, with a stop in Naxus.
Boat was also a detail in Samos, allowing us several hours in Turkey, to visit Ephesus from Kusadasi.