Getting to the Acropolis from cruise port via metro

This may provide some help

https://www.athenstransport.com/english/

We used the X80 bus from the port into Athens Very easy.
An express bus service introduced in 2015, X80, provides service between the cruise terminals and Syntagma Square with only four stops along the way including one stop near the *Acropolis*. Ticket price is €4.50 for a 24hr transit pass available from the driver. Buses operate from 7am till 9:30pm, and run approx every 30 minutes. Service should begin in May for the 2019 cruise season. Route and timetable here https://www.athenacard.gr/
Taking the metro is also a convenient way to get into central Athens. The metro station is a 20-30 minute walk from the terminal following the harbor. You can take a bus but they don't sell tickets onboard but at kiosks, newsstands and mini markets. From cruise terminal A, head north crossing the main street and look for a newsstand to buy tickets. The bus stop is nearby across the side street. If you can't find it or if they are sold out, keep on walking north towards the metro station keeping the harbor on your left to find another one. Tickets are good for 90 minutes (valid for buses, metro or tram) and only €1.40 (Mar 2019). Bus stops should be signed for the 843 or 859 bus (google maps have the bus stops marked). You will need to validate your ticket onboard by time stamping your ticket in the machines. Ask the driver to stop at the metro station or you can spot it by looking for the pedestrian overpass. https://www.athenacard.gr/
If you've decided to walk all the way, you can buy your ticket at self serve machines at the station when you enter but staffed service counters are to your right. Remember to validate your ticket before boarding. If you plan on doing more than three trips, it is more economical to buy a day pass (€4.50, Mar 2019, not available at newsstands). You will probably get off at Monastiraki, Syntagma, or Akropoli station (last two require changing lines).
Coming back, use the pedestrian overpass to cross the street where you can catch the 843 or 859 bus back to the terminal.

1. Walk to Piraeus metro station. Check Google Maps for location, and ask when you get off your ship.
2. Buy a ticket in the machine at the station.
3. Take Line 1 (Green) to Omonia; transfer to Line 2 (Red); Take Line 2 to Akropoli. When you get out of station, turn left and there it is.
4. Or if you don't mind a short walk, get off Line 1 at Monastiraki and walk about 500m to the acropolis. Map: https://upload.wikimedia.org/wikipedia/commons/4/4b/Public_transport_map_of_Athens.png

Our ship docked at the furthest berth in the second terminal area in Athens. It was about a 30 minute walk to the metro line. If you are in terminal B it adds a lot of steps to your day in Athens.
You must also be aware that pickpockets operate on the metro and stations. They are not dangerous but they are quite skilled. Take precautions to secure your valuables.

We did it back in 2010, others have explained. It is not hard. Just wear a money belt. Pickpockets are terrible in Athens.

The Acropolis Station :) It's around 40 min from the port if you take the tube