Travel Note
From the airpot go to the taxi zone, (outside of the airport) grab a taxi to take you to the port (Piraeus). The drive is approximately a 60 min ride. Once at the Port you can get the ferry that goes to Hydra.
Travel Note
This website allows you to book your ferry to Hydra online, once you reach Piraeus, go to the ticket office and take your ticket. You will find them inside the port on the edges. Go to the ticket office that has the name of the company you booked with (Flying cat, Alphalines, Magic Sea). You can also book straight at the Port without using the website.