Ermioni is located in Peloponnesus, right opposite Hydra, 180 kilometers far from Athens. To get there, you should follow the Motorway from Athens to Korinthos, and leave the motorway in the exit towards Epidavros, and then follow the signs to Ermioni-Kranidi and then Metohi. See here the route.
After Metochi, you will cross the sea to Hydra on a speedboat ("FREEDOM" ships)or on a sea taxi.
In Hydra cars and motorbikes are prohibited