Interesting links  
Car rental in Porto Heli:  
Pop's Car  
Car rental at Athens airport: Budget
Avis Car Rental Hertz
Sixt Europcar
Pop's Car Kosmos Rent a Car
General websites on Porto Heli:  
VERVERODA LAKE WaterSki Centre Most correct weather forecast (in greek)
Porto Heli Scuba Club (for divers) Marine Tours Port Heli (boat trips)
Frank's Yacht Station (for sailors) Die Argolis (german)
Motivas (rental of  Beach Buggies, ATV's,  Waterscooter)  
Ferryboats to Greece:  
Minoan Lines Superfast Ferries
Anek Lines  
Ferryboats from Athens to Porto Heli:  
Hellenic Seaways