The ferry Newcastle to Amsterdam (actually to IJmuiden, about 40 min bus ride from Amsterdam) is an interesting way to travel if you have the time. It's perfectly comfortable, especially if you get a nice cabin. Don't go expecting luxury! Good summary of the service from the
Seat61 website.
It departs early evening (about 5-6pm usually) which would combine well with a leisurely departure from Edinburgh and a few hours exploring Newcastle, then dinner on board.
If you were in Newcastle, I wouldn't hesitate to recommend it. Starting from Edinburgh, you'd have to be looking for a slow travel experience to recommend it over the direct flight.