https://benefitsportal.eurail.com/?categories=1be1ba1c-80f5-4286-b255-3092f0d486cd

All ferries that offer discount or are for Free. 

Thanks mate

If its ONLY discount-then you do not need to use travelday (if you hold such a pass). Most of such ferries will offer discounts to any ju/senior. There are in fact even more as stated on the list-and others that are stated are reported to not offer it anymore. ALWAYS book on the site of the ferry concerend for that-not via the dozens of general ferrybooking-sites.

IF its ´free’you will still have to pay for various taxes (f.e. from It to GR quoted as at least 32€) and also use a travelday. A cheapo advance ticket might then be well, uh, cheaper!

Due to current very high diesel prices most ferries add a fuel-surcharge.