Sea sickness is a form of motion sickness. It strikes some people on rough days at sea on a cruise.
The over-the-counter medicine meclizine (in the brand names Bonine and Antivert) is commonly used and often effective to prevent sea sickness or to treat after onset. Dimenhydrinate (Dramamine) is another drug commonly used.
This video by Cruise Tips has advice for how to prevent motion sickness. It is less than 5 minutes long.