Also known as DFSD Seaways.
Travel the following routes:
Newcastle to Ijmuiden (Holland) for Amsterdam - 16 hours
Newcastle to Bergen (Norway) via Stavanger and Haugesund - 27 hours
Newcastel to Haugesund (Norway) via Stavanger - 24 hours
Newcastle to Stavanger - 20 hours
Harwich to Esbjerg (Denmark) - 16 hours
Copenhagen to Oslo - 20.5 hours
This is an efficiently run company with sturdy reliable ships. Make sure that you eat well as soon as you get on board to avoid risk of seasickness. Prices are competitive although not especially cheap, but the main reason for going by boat is to savour the journey, which you can't do so much in a plane.
Really worth looking out for are the special deals you can get sometimes, which might be a cabin upgrade, or free accommodation for a short break.
It's also worth paying for a good cabin, as the walls are quite thin. Onboard entertainment is fair, if you like that wort of thing.