Carpacchio's idea is fine but, unfortunately, there are no ferries from Piraeus to Porto Heli. Thus, a reliable way to get there is a KTEL bus to Nafplio (a very, very very beautiful city, Greece's first capital) 2.5 hours from Athens and then a cab to Porto Heli (~80 km). Alternatively, and certainly nicer, a ferry boat or hydrofoil from Piraeus to the island of Spetses (probably what Carpacchio had in mind) and then a boat-taxi or another local boat transfer to Porto Heli. If you think of the transfer as part of your vacation, no problem.