The DiscoverEU Interrail Pass includes access to several free and discounted European ferry routes. Here you can find an overview of the major ferry lines that have a partnership with Interrail.
Whether a travel day is needed depends on the discount rate of the total fare for the ferry. If the ferry is fully included in the Interrail Pass, you will need to use a travel day to take this service. If the ticket price is discounted, but you still need to pay a portion yourself, then your Pass only needs to be active (meaning your 1-month validity period has started).
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, simply check out the description of the ferry you'd like to take here. 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.