You can travel with car from Athens to Hydra island in approximately 3 hours and less, by following Attiki Road and then the national road Athens-Corinth. After the Isthmus you turn left towards Epidavros (Corinth Exit B Epidavros)
Drive for 30 minutes and reach the intersection towards Epidavros. You will also see quide posts for Poros or Spetses. Turn right to the intersection and under the bridge.
You follow the newly constructed road watching out for the sign writing Poros. There, turn left to Poros and reach Galatas from the coastal road in 30 minutes. Then continue through Plaka-Lemon Forest-Aliki and after half an hour you are in Metochi, where there is a parking space. From Metochi you travel across to Hydra.
From Kranidi, Porto Xeli, Kosta or Ermioni, you reach Metochi by following the coastal road towards Thermisia in 45'.