Athens airport to ferry port timing?

You did not specify which day in April you will take the ferry.
But apparently the only days with a ferry to Hydra at 12.45pm and the next one at 3.15pm are April 14 and 15 during the Orthodox Easter weekend.

It's a busy time with heavy traffic on the roads, airports, and lots of people taking ferries (these ferries are added especially for Easter).

45 minutes being the minimum time needed in normal times to go from the airport to Piraeus by taxi, it would be wiser to book the 3:15 ferry.
Tell the taxi that you are taking a ferry to Hydra. Departures for the Saronic Islands are from Gate E8 in the port of Piraeus.

Should you ever be able to arrive early enough to catch the 12.45pm ferry, you can always change your tickets on the spot at a ticket office.

Is Athens your entry into the Schengen Zone?

The answer to that question has a meaningful impact on how much time you need at the airport, before you can leave.

Sorry - we are coming from Atlanta - and yes, arriving April 15.

Sounds like a 12:45 is not realistic?

Sounds like a 12:45 isn't realistic?

If your flight is on time, passport control is done quickly, you meet your taxi driver right away, there is no traffic jam on the way to the port, you will probably be at the port no more than 20 minutes early to catch the 12:45 p.m. ferry.

It's up to you whether you want to take the risk.

It's not unrealistic, it's just that there are a lot of "if".

I’m hoping to buy the ferry tickets soon.

If you really take the regular ferry and not the hydrofoil then no reservation is necessary, because on the ferries is always space for foot passengers (it would be different if you had a rental car). So you don't have to make a reservation and you can just drive into Piraeus and buy your ticket at the spot. Then there is no problem either with the ticket if you go with another carrier than the planned one.

Thanks to all - this was very helpful!

After a redeye, I'm not thrilled about the idea of either missing the 12:45 ferry (which certainly seems possible) or spending three hours waiting (and then having another 90 minute ferry ride) ... so we are thinking about shifting to Aegina instead (more ferry options, shorter ferry ride).

Anyone have advice on Aegina?!?

Thank you all so much!

Aegina's a better experience anyway -- because Hydra ferries are enclosed hydrofoils with salt-glazed windows, no view .. whereas for Aegina you also have a choice of conventional ferry (just 1 hour ride), where you can sit out on open deck & enjoy the seascape -- huge freighters & cruise ships, sleek speedboats, 3-masted schooners, on a fair day, even tiny sloops. Here's a great photo-sequence of a JUne daytrip - http://www.flickr.com/photos/36264706@N03/sets/72157621604646139/detail/ -- which shows some of t he highlights of the island.

Hydra brags about having no cars, but this also has the disadvantage of limiting how one gets around... riding a donkey? Climbing endless steps & lanes while jet-lagged? Not so fun. Aegina has busses (fewer, off season) cars & taxis, but I can testify that off season (even in late May during week, when I went) traffic is very minor)... I walked down a road for 15 minutes an no car went by. One can rent a car, but lately I hear that you now can also rent E-bikes, which are safe, reliable pedal-bikes with motor-assist... which means u can pedal if you wish, or just use the assist, and put-put sedately & safely along.

In April, I'd advise finding a nice hotel in the port town since the beach areas while great to visit, & stroll the sands, will just be "waking up" for the season, and likely won't yet have many tavernas & cafes open, unlike the port. PLus, the back streets of the town are fascinating, not touristy.. I love browsing in groceries & housewares stores just used by locals, seeing different brands, foods, decor items. HYdra is more for celebritiies, and for their fan clubs, where Aegina is the fun getaway for Athenians (and for some, the suburb, many residents commute daily for work)O.