Your InterRail Pass entitles you to discounted or even free ferry crossings for some European ferry services.
If you travel with a Flexible Rail Pass (XX travel days within a time frame of YY days), the following rule applies:
If the ferry departs after 7 p.m. and arrives after 4 a.m. at the destination port, you only have to use ONE of your travel days. Write the date of your arrival at the destination port on your Rail Pass.
When traveling by night ferry (Italy-Greece: Super Fast, Minoan or Blue Star Ferries) you can fill in either the day of departure or the day of arrival. Please note that you cannot start travelling before the first day of validity on your Pass.
For continuous rail passes the travel days do need not be specified.
It is specified for each ferry connection how to book or buy tickets.