How long is the ferry ride from Santarini to Athens and what time (ish) do they depart Santarini for Athens?
I did not see alot posted yet for the main season, and of those, only the slow Bluestar ferries. Those that show are overnight (leaving late evening) or leaving early morning. These take 6-8 hours, there are some fast ferries that might cut that down to four. My preference though is flying. Aegean Air has about four flights a day, as cheap as 50 euro, typically 100 euro for a one hour flight.
The conventional Blue Star car ferry takes 8 hours. The highspeed Hellenic Seaways catamaran takes 5 hours. So far the Blue Star is the only one to have posted their schedule. There is a daily 7:30 AM departure. Hellenic Seaways will post their schedule about a month before it's needed, perhaps sooner, but most likely not before mid-April.