Fast ferry´s, normal slow car ferry´s ( cheaper ) or even a bus from Athens bus terminal to Galata.
Galata is just 250 yards from Poros.
Hire a car in Athens, drop it off in Galata or Poros.
Yep I concur with the above as i used to regularly go to Porto Cheli ( around the corner from poros) where we kept our yacht before moving to marmaris. The bus to Piraeus from the airport then one of the fast ferries/dolphin.