There are lots of things you can try to prevent motion sickness. Some will work better for you than others - every person is different, so you are likely to get lots of different opinions here. Ginger tablets/capsules are commonly used, as are special wristbands that use accupressure. I know some people that swear by them. They do not, however, work for me, but over the counter Bonine or non-drowsy-formula Dramamine work great, and I feel fine, not too sleepy to enjoy the cruise. (This has worked even in rough seas.) Or you can get a patch prescribed by your doctor.
Location in the ship can make a difference too. Lower decks are generally better, and amidships (toward the middle of the ship) also. However, remember that you will be eating in the dining room or buffet, which are usually on higher decks (the buffet near the top), and if you want to enjoy other activities, they will be often be found higher up on the ship. Therefore, I recommend trying the medication(s) and/or wristbands. I am sure you will find something that works for you - I can get sick in the back seat of a car, so if I can feel fine on a cruise, you know it's possible!