Sometimes you can feel the motion in the back of the ship, it is usually worse in the front. You are going to the Inside Passage which is one of the smoother places to cruise seeing there is land on one side and islands on the other. The roughest part of the cruise will be before you hit the Inside Passage and once you leave it.
As far as the motion sickness goes I get motion sickness easily and I have never gotten sick on a cruise yet. There are many things you can do to help prevent it. Some people take ginger tablets and swear by that. There are pressure bands that you will see some people wearing on their wrist. Dramamine or Bonine are used by many. You can buy a non drowsy dramamine if you worry about getting tired from the dramamine. You will see many people with the patch which is a perscription from your doctor. Beware that these sometimes effect people with high blood pressure.
A good walk in the fresh air helps and believe it or not having something light to eat can also help. It sounds odd but it does work for some.