If you are prone to seasickness obtain some anti nausea medicine,but check with your doctor.The ship is huge with many comfortable lounges and cabins.A cabin is nice to have as one can lie down if feeling sick.Keep your eyes on the horizon,and they say eating something salty helps.I took the route and felt seasick when departing Prince Rupert after 45 mintutes entered the open ocean for about three hours.Also if you take some bitters,non-alcoholic kind it helps.

We did the Inside Passage ferry trip in the aftermath of a storm, in early March 2007. During the storm, the boat was stuck over on the Charlottes because the seas were too rough (IIRC, seas over 6 meters) for it to get to P.R., and so our trip started fourteen hours late.

Most of the trip was just fine, very calm, because the straits are very sheltered, but there were one or two spots where we had to traverse more open water where it was rougher. I am not really prone to seasickness, but at one point I was feeling just a little "off" for a few minutes.