Interrail Passes include access to a number of free and discounted European ferry routes. Here you can find an overview of the major ferry lines that have a partnership with Interrail.
It is often possible to purchase special fare tickets for ferries in advance. It is recommended to do so when planning to travel during the high season. To purchase these tickets, you will have to contact the ferry company you will be travelling with directly. In some cases, you can buy the ticket with the discount already included. In other cases, you will have to buy the full fare ticket and the ferry company will refund the discount.