Rafina to Mykonos is the shortcut to Mykonos for anyone landing at Athens airport. Rafina is a small harbour town on the east coast of Attica, much closer to the airport than Piraeus, and it has more departures to Mykonos than many people expect: on a peak summer day boats leave from about 07:00 to late evening (Ferryhopper lists a usual last departure at 23:00), with a mix of fast catamarans and slower car ferries.
Conventional ferries usually call at other islands on the way, which is why the slower boats take three to nearly five hours. At the far end you arrive at the new port of Mykonos in Tourlos, about 2.5 km from Mykonos Town, with buses, taxis and hotel transfers waiting.
In a trip plan, Rafina works best if you fly into Athens and want to reach an island the same day, or if you are starting in the northern Cyclades. A popular pattern is Rafina to Mykonos, then on to Tinos, Naxos or Paros, and back to Athens from one of them.
Before you book, weigh up speed and price, though fares overlap more than you might think: conventional ferries start at €38 and carry cars, while some Seajets sailings are cheaper still, from €35.50 in Ferryhopper's table of last year's data. Return sailings to Rafina fill up on summer Sunday afternoons and at the end of weekends, so book the way back early too.