Does anyone know anything about Greek Ferries?
The links provided by the others show the timetables. You will see that there are several ferries going each day. I have often traveled from Rafina to Mykonos or Tinos on these ferries and have never had a advance ticket. It has never been a problem. For your own peace of mind get to the port well in advance so you have times to buy your ticket, and if by any chance one ferry is sold out, you can always take the next one. I am CERTAIN however it won’t be a problem. (The only instance where there may be difficulty getting a ticket is if you are traveling around the 15th of August. The reason for that is that the island of TINOS, which is on the same route as MYKONOS, has a huge religious celebration at that time…..and thousands of visitors from all over Greece flock there for the festivities.