We've done this crossing many times. If you are travelling at night you will have a cabin. But if it is a day crossing, you can have a cabin but I think it costs a bit extra also nice if you want to have a nap! If its not very busy I'm sure you will find a quiet area with not many people around. There is a shop, cinema and a couple of restaurants and bars and plenty of seating areas. Enjoy the crossing.
I think I went overnight too, but having the cabin was worth it - there are quiet areas but not really private areas as such - mind you I wasn't looking for changing areas etc so I might be wrong. If you can get a cabin during the day, adn I think you can though it's a bit extra, I'd go for it. For what it's worth I loved it, beats the train anytime