We are going to be going from Monenvasia to Hydra and on Rick Steves map it looks like there is a ferry from Limani Geraka (or from Plaka or from Paralia) to Porto Heli. I thought that might be more fun than driving since we will have been driving a fair amount before that. Does anyone know what ferry company that might be?
I did see the RS map as well. Ferries change frequently so its possible that it isn't running anymore. From all the resources I found online, I couldn't find one reference to a boat from Limani Geraka to Porto Heli. It looks like the only boats that go to Porto Heli are coming from Ermioni, Hydra, Piraeus, Poros, or Spetses, not Limani Geraka. Plaka, or Paralia. You may have to drive a bit further up the coast to connect with Ermioni, Porto Heli, or Poros in order to connect by ferry to Hydra. You can find ferry information on www.aferry.com and www.greekferries.gr. I hope this helps some.