Athens airport to ermioni or thermisia

That’s a long distance, so taxi is not going to be cheaper. Most probably do it by rental car. Have you determined how you will get around without one?
There are buses to Nafplio, so you could research onward travel by public transport.

Thank you for your response. I can’t rent a car as I am going to a retreat in Thermisia then to car-less Hydra before returning to Athens airport.

To get to Ermioni, which is still not my final destination, I was told I need to take 3 buses.

From ermioni to thermisia, a short distance, how easy would it be to get a cab or uber at night?

I doubt that you would get a much lower price, because one would need to pay for a round-trip for such a driver, and what would amount to a day's worth of his time. Do you often use Rome2Rio? https://www.rome2rio.com/map/Athens-Airport-ATH/Ermióni#r/Bus/s/1 that's a good place to start ... it shows that you can get the x93 KTEL express bus from the airport to the Kifissou intercity KTEL bus terminal for €6, takes about 1 hour, and then an intercity bus to Ermioni for about €8, takes 2.5 hours. The problem is -- usually only 2 busses per day... 9:30 am, 4:30 PM. If that works with your schedule, you're in luck.

The bus Janet is talking about is the KTEL Argolidas bus. Bus schedules and fares below:

https://www.ktelargolida.gr/en/kranidi/

If the bus schedule doesn't match yours, you can also take a bus to Nafplio and then a taxi.

Have you contacted these taxi services in Ermioni?

Korontinis:

https://taxiermioni.gr/radiotaxi/en/taxi-ermioni-nikos-and-dimitris-korontinis-24hour-service/

Develekos:

https://644c33bfbdf08.site123.me/

Fanouris Aggelis:

https://taxi-fanouris-aggelis.webnode.gr/epikoinonia/

Ubers are regular, official taxis. In Greece it is forbidden to practice the taxi profession without a taxi driver's license.

Thanks so much for your replies!

1. Is it important to reserve a taxi in advance at Ermioni or is it possible to hail at the time of arrival?

   1. Do folks know of any high speed ferries leaving from Athens to ermioni and would you recommend those instead of the bus?

I guess you could get a ferry at Pireaus to Hydra (Idra) and get a bus or taxi from there to Ermioni. A KTEL bus from Airport to Pireaus takes a bit more than an hour, I think goes every 40 mins? you'd have to check online.

The Retreat host doesn't have info on how to get there?

A private transfer such as this --- https://www.suntransfers.com/. ( plug in your date and arrival time to see a cost)
shows ~400 euro for the trip to Thermisia ( up to 6 passengers) in a van.
Can you set up with other attendees to share a van or car?

A car for up to 3 passengers is ~ 300 euro

I doubt a taxi/uber will be as low as 250 euro

The Piraeus to Hermioni routes are operated by Hellenic Seaways ferries (high-speed catamarans).

Depending on the ferry and its departure time, the journey time can vary from 1.5 to 3 hours, as some ferries stop first at Hydra and/or Poros, while others do not. Regardless of the number of stops, the ferry ticket to Hermioni costs €46.50.

The metro journey from the airport to the port takes 60 minutes; there is a metro every 36 minutes starting at 5:30 a.m. Ticket price: €9.

The arrival and departure point for Hellenic Seaways ferries to the Saronic Gulf islands is at Gate E8, located about a 15-minute walk from the metro station.

Check schedules on the Hellenic Seaways website:

https://www.hellenicseaways.gr/en-gb