The Railplanner just show a connection :)

Almost all ferries need a booking or surcharge :/

Even the “Free” ferries from Italy  to Greece charge a port tax and fuel surcharge :/

On the Baltic Sea you only get a discount on the ferries there is no “free” ferry in operation

Here you can check the Ferries that offer Discounts for Interrail and Eurail https://www.interrail.eu/en/plan-your-trip/trains-europe/ferries

It’s not true that the Railplanner just show a connection, as you can see from the screenshots, it says if you need a seat reservation or if there is a surcharge...

@Davide  the Railplanner is not always correct

It shows for example even not the Surcharge for Eurocity/Railjet from Germany/Austria to Italy :) 

and the Railplanner shows even trains that are not covered by the Pass on most of them is written not in PassNetwork but not on all :)

and as said allmost all feries have to be paid (even the “Free” ferries charge a Fuel and Harbour Fee )

And Interrail provides all the Informations on their Pass Benefits section https://www.interrail.eu/en/plan-your-trip/trains-europe/ferries
 

It´s quite hard for Eurail to keep all Informations on their Websites and Apps actual as usually they get no notice from the Companies For example DB stopped silent all their IC Bus Services or in Poland some regional routes are taken over by new regional traincompanies that are not in the Passnetwork.

Eurail tries a app that covers entire Europe in my experience they are the only one. All others offer Route planning for just selected Countries. Sounds stupid but before useing a pass benefit like Ferries or for example the free Ships on Lake Brienz / Lake Thun i checked the websites or asked the operating Companies

As far as I can see, a discount would not apply to this departure:

https://www.interrail.eu/en/plan-your-trip/trains-europe/ferries/viking-line-ferries