When choosing the accommodation, you can choose your pass, e.g.:

This is on Superfast.com.

You must use their own site-as above, as if you use google-there will appear a dozen or more of general ferry-bookers. It may pay off to compare though-another one even without discount may be cheaper in he final price.

Hi, 

I’ve got the interrail global pass too. 
 

However I’ve found that the global pass discount is only for Adults (25 - 59). 
 

As of this moment I’m 20 yrs old and is considered a juvenile when booking. 
 

what advice do you guys have for getting the ferry discount? 

Choose Adult? Since that gives the biggest discount. The youth discount is less than the pass discount.

Otherwise, mail the ferry company and ask them to book a reservation because the website won't let you because of your age.

Hello!
Do you know how much the price would be right now with the Global Pass?
And does the ferry trip count as one or two travel days?

Hi!

I am having the same problem: I can only get the Interrail discount if I choose “adult” as my age. However, I am still a “juvenile” (22) for Superfast. If I choose adult, will my age be checked for example from my passport? :) 

Use https://www.anek.gr/en/ to book. Prices are the same but they seem to have fewer bugs in their website.

Thank you! However, I cannot find the destination port I need from them. :/