Do you find that the flights are always full from Athens to Milos?  
Then take the boat !!

Because of the shortage of seats on the Olympic Aviation shuttle we recommend that you consider another form of travel that is currently being extended. Getting to Milos from Piraeus is easy using either a high-speed catamaran or one of the fleet of hydrofoils both of which take 3 - 4 hours for the journey. They are equipped with aircraft type seats and have air-conditioning. Taking into account the time spent to reach Athens airport, the one hour check-in, flight time and baggage retrieval at Milos airport the sea connections can be just as fast as going by air. These connections operate to a schedule, the frequency of which is changed according to the season. Only in the worst of bad weather are the sailings suspended which is hardly ever during March - October.

On arrival at Athens (Spata) airport take the express bus E96 marked "PEIRAIA"  At the harbour you will find the ferries to your right and the hydrofoils and catamarans to the left. Click on 'Piraeus' to see a map of Piraeus port. The price of the bus ticket is GB£ ~0.4 and the fare for the 'Flying Dolphin' is about GB£30 (much cheaper than the plane) The ordinary ferry costs approx. GB£10 one-way and takes about 6-7 hours calling in at 3 or 4 other Cycladian islands on the journey to Milos. Tickets for the ferries are sold at prominent kiosks on the harbourside (see plan). It is not possible to make reservations but there is never any difficulty in getting a passage. Should your schedule mean an overnight stay in Piraeus open the page 'Piraeus' where you will find helpful information 

A useful  advice
You can book and buy your boat tickets to Milos directly in the port at the travel agency Filippis Tours. The  office is only  150m away from the arriving point of the X 96 bus from the airport and the taxis  and next to the  cafe-restaurant  FILOXENIA (see red arrow on the map)
Mr Nikos Kiosoglou, a good friend  who speaks many languages is always very helpful. You can either  book  your tickets on line http://www.filippistours.gr or send  your  request by mail  and pick up your tickets  on arrival just before  your ship departs. Working hours 06.00-22.00  daily. 

Timetables (See Schedules)

I show below how to find which boat to take for Milos with the days and times. If you click to the left on the logo "GIH" it will ask you to insert the code name for the port of Piraeus and also that of Milos (Adamas). As there are many ways of spelling Greek place names I suggest you enter "PIRAEUS" or the code "TZE". For Milos there is no change, it is "MILOS" or the code "ADA". As you continue your search you will see a list of the sailing times and you will also receive details of the boats and the ferry operator etc. Ferries operate all year round.

A new and helpful link is www.milostravel.gr/uk/dromologia.htm.  Here you can find the boat connections from Athens to Milos and back and also to all other islands.