I agree with Ginger being taken. The Pharmaceutical products available are really of no benefit however natural remedies may help
A few other tips to help prevent sea sickness motion sickness are;
Being anxious or worried about getting sea sick it has been found actually contributes to the problem. Some people even feel nauseous before the actual trip has begun. So try and relax before the next trip and don’t think about getting sick.
Watch your consumption of foods, drinks, and alcohol before the trip. Avoid excessive alcohol and foods or liquids that "do not agree with you" or make you feel unusually full. Heavy, spicy, or fat-rich foods may worsen motion sickness in some people
The saying "watch the scenery go" by is very apt in your situation. If you fix your gaze on the horizon helps don’t look at other boats on the water or the waves or anything that has a rolling motion
Try to keep your head still as movement of the head moves the fluid in the ear canals around and it add to the “sensory confusion” your body is experiencing
Try to sit or stand where you will experience the least motion and do not sit facing backwards from your direction of travel
Do not read when travelling if you are prone to motion sickness
Wherever possible make sure you are in the fresh air away from any cabin fumes etc