There are several different things one can do for motion sickness. Everyone is different in their sensitivity to motion. Remedies can be as simple as eating ginger or green apples, keeping food on the stomach, and when up on deck focus on the horizon. They may speak with their Doctor about a medicated patch which is placed behind the ear. Some people take the Bonine motion sickness pill. There also are some wrist bands you can buy which put pressure on pulse points.
The ships all have stabilizers. You may want to go on one of the newer mega ships. I can hardly tell they are moving. You may want to book a cabin midship on a lower deck also.