Seasickness or motion sickness is caused by excessive stimulation of the vestibular (ear) apparatus and seems to affect some people more that others.
Aggravating factors are visual stimuli (such as reading or a moving horizon), fumes, smoke, poor ventilation, perfumes, anxiety, alcohol or a heavy meal before travelling.
Stimuli such as a moving horizon are hard to avoid while on a boat, but try driving or steering the boat yourself if possible, this will help as you focus more on horizon which is stable.
Homeopathics containing Nux Vomica, Cocculus and Pulsatilla are excellent, best taken several hours before travelling and then every hour or so as needed.
Charcoal tablets can help settle the stomach, as can ginger.
Be sure to drink plenty of water in small amounts, and eat simple foods, frequently. Dry crackers are a good idea.
Personally the best remedy I have ever found…is a very bitter beer. I find that the bitterness helps settle the stomach and the bubbles bring up any stubborn gas!!