After you arrive at Piraeus either by bus or taxi, you will catch a flying dolphin (hydrofoil) or cat (catamaran) to Hydra. The trip takes in the neighborhood of 2 hours, typically stopping briefly in Poros, making Hydra the second stop (if the boat travels direct from Piraeus, the trip will be 1.5 hours). Currently, Hellenic Seaways (Blue Star Ferries) operates this route.
Please note that you often won’t be able to book your tickets until about a month before your departure, as the schedules have not yet been posted. Those traveling with heavy or awkward luggage may find it most convenient to try to book a reservation on a flying cat. These larger catamaran-style boats are far more spacious, and boarding and debarking are much easier. Getting on and off the flying dophins can be a little more challenging. Travel on the cats also tends to be more comfortable, and both a business class and food and beverage service are available (not available on dolphins).
Reservations are highly recommended in the summer months, particularly for weekend travel. Many Athenians have weekend homes on Hydra, so boats on Friday and Saturday tend to fill up quickly. The same holds for return boats to Piraeus from Hydra on Sundays and Mondays. If you can’t make your reservations via thewebsite (likely because the seasonal schedule hasn’t yet been posted), you can try booking by phone.
Hellenic Seaways website (+30 210 4199000)