Hi,
I went on the regular ferry from Hook Van Holland to Harwich on a night crossing a couple years ago. I am really glad I paid a little extra for a cabin. As previously posted it was very smokey. Most of the passengers were from U.K. and spent the night drinking, gambling and smoking. My wife smokes, I don't. She loved it.
If you are a non-smoker and don't go high speed, spring for a private cabin.
Steve