Do I need anything besides my Greek Islands Pass to board the ferries?

Domestic routes: No, your e-boarding pass is included in your e-reservation. Therefore, you can hop on board the ferry by simply showing your e-boarding pass, and Eurail Greek Islands Pass 

International routes: Yes, you will need a boarding pass for each ferry, which you can pick up by presenting your reservation at the embarkation port. On international routes, it's mandatory to pass through the port to show your passport or ID to the authorities. 

We advise you to be at the port to board the ferry about two hours before departure.

Making free reservations for each ferry is also required. You should reserve your seat or bed via Attica Group at least one month in advance, especially for sailings during the busy summer period.