I was able to get from mainland Athens to Greek islands via the Greek island ferries, but I recommend catching a flight.
Because one should arrive at the airport well in advance, and it’s a trek to the airport, I figured the ferry would technically take the same amount of time.
If you insist on trying out the Greek island ferries, which honestly make sense depending on where and how far you’re going, then I recommend purchasing directly through these websites:
I paid 174.90€ total for ferries from Athens to Santorini, Santorini to Mykonos, and Mykonos to Athens.
Where to Stay in Athens
Even though the journey was a long one, I’m so happy that I visited Santorini during my time in Greece.