Forgot to say: The ferries are great and most of the fast ones leave from Rafina which is very close to the airport making for an easy alternative. However, you know that wind that blows through the Aegean during the whole of late July and August (and pretty much goes away from 1st September)? The same wind that on a bad day sand blasts you on Elia/ Super/ Ornos beach making it uncomfortable to sit on the sand (and a pain to shower afterwards). Well, that same wind kicks up some pretty rough seas and the first time you get caught on a fast ferry to Athens in one of those will likely be the last time you take the cat/ ferry. At least it was for me.
So yes, the boat is a good option but take that option with caution. And if you're connecting at the airport anyway, then might as well fly.