By Car To Hydra Island
Drive, but only as far as Metochi or Ermioni on the Mainland because no cars are allowed on Hydra Island.
Landing at Athens Airport and picking up a car rental so you can drive yourself across the Peloponnese and down to Metochi or Ermioni couldn't be easier.
It's impossible to bring a car onto Hydra because of the islandwide ban on motor vehicles. So however close you get by car, you can't avoid a passenger sea trip.
This video by Nikos Sorras shows the quickest route going via Thermisia. I strongly recommend you watch it to the end so that you notice the dog!
RentalCars.com
have a great site where you can choose from all of the hire companies based at the airport with prices starting from €9 per day.
This gives you the option to explore more of the mainland and the choice of parking at Metochi or Ermioni to catch a passenger ferry (25 mins) to Hydra.
Don't forget you will have the cost of a full tank of petrol (enough for a round trip without detours) plus the daily cost of parking in the secure car park at Metochi. (You can park on the street at Ermioni.)
From Athens (or the Airport) take the National Highway to Korinthos, then follow the signs for Epidavros. Then make left and keep going for 30 minutes, following the signs from Epidavros to Ydra, Poros and Spetses, you'll do a right and left under the bridge.
After the bridge you have two routes to choose from; one goes through Galatas and the other via Ermioni.
The Galatas route takes approximately 25 minutes and from Galatas Town follow the signs along the coast to for another 30 minutes to reach Metochi. You will turn left off the coast road to the Metochi Ferries for Hydra.
See the video at the top of the page. If you choose Ermionis way, after about 40 minutes you will reach a road sign (Ermioni-Kranidi-Porto Cheli) at this point you have to follow Ermionis direction and at the next sign that says Ermioni - Thermisia you turn off and follow the sign to Thermisia, drive through Thermisia and 15 minutes along the coast you turn right to the Metochi Ferries.