Reservations are usually recommended to make sure you get a place on the boat. In general, reservations for boats can only be made at the ticket window of the particular ferry line. Sometimes, you can make reservations online through the ferry company's website, or via phone or email. The individual ferry company descriptions will tell you the preferred method of booking.
Take a look at the page Extra pass benefits for an overview of free or discounted shipping lines with your Eurail pass.
