Ferries are an important means of transport on many Eurail trips. They complement existing train lines and allow shortcuts across the sea where trains would have to travel much longer.
A number of ferries is included in Eurail (port and fuel taxes might have to be paid separately) and many ferries offer discounts for Eurail pass holders. If a ferry is free with Eurail you have to fill in a travel day. The 1900-rule for night trains applies for overnight ferries too which means that only one travel day is needed for overnight trips. Unlike for overnight trains it is even possible to fill in the departure day if it suits you better. For ferries which grant a discount for Eurail you do not have to fill in a travel day.
The free trip included in Eurail or the offered discount usually applies for the standard deck passage. Additional costs such as berths or cabins, food, fuel and port taxes have to be paid separately. Free and discounted tickets are usually available directly at the ports, most ferry companies also offer booking of discounts tickets online or via phone. For more details about timings, fares and how to buy tickets look at the information about the different ferry companies and routes.