You must have access to a couchette or bunk. A couchette isnt a bunk in private cabin, but it is more than a reclining seat.
Short crossings dont have bunks as standard, but, If they are there, why wouldnt that count?
I cant remember seeing any rule about a minimum crossing length for daily rest interruptions. "A regular daily rest period may be interrupted no more than twice, but the total interruption must not exceed 1 hour in total. This allows for a vehicle to be driven onto a ferry or train and off again at the end of the crossing including customs formalities." It goes on to explain about a split daily rest counting as a regular daily rest, and mentions 3 hrs there, but I dont see that as relevant here.
Barry is spot on about 8hrs min for interrupting weekly rest.
“EU weekly rest periods may be interrupted no more than twice, but the total interruption must not exceed 1 hour in total but when interrupting a regular weekly rest period, the ferry or train leg must be at least 8 hours duration”