Ferry Tickets to Hydra Greece from Pireaus

There are kiosks at the port in Piraeus. Determine which boat you're on and which dock (by number, like E3), and then there will be a kiosk nearby. You bring your voucher or printed verification and ID

Look up the port on google maps and zoom in. Dock numbers are printed on the map:

https://www.google.com/maps/place/Piraeus+Harbour/@37.9443604,23.6329256,16z/data=!4m5!3m4!1s0x14a1bbda6f0d2661:0x5fb5818ce4535dda!8m2!3d37.9405556!4d23.6333333

Thanks! That should be simple enough. I guess that's why they say to get there at least an hour prior to departure.