One gets seasick, the other doesn't. Some only sometimes, others more often. Why is that? It is largely predisposition. It depends on the weather, I've seen more people vomit when there is no wind than when there is strong wind.
Recipe for seasickness
Go to bed early in the evening, little alcohol.
Have a good breakfast.
Drink enough and eat again and again on the way.
This recipe worked in most cases. In many cases where it was ignored, only the bucket helped the next day at sea.