We never know for sure and people do get seasick.
Seasickness can be avoided in most cases if I have the cooperation of my passenger. I have never had any one get sick while fishing on the bay yet! When offshore, you need to alert me to the first signs of feeling uncomfortable, which I usually notice before you say anything. With a quick return to bay fishing, seasickness can usually be avoided.
Seasick pills can be taken a half hour before getting on the boat and you can get seasick patches by subscription. It would also help if you do not over indulge the night before and get good night's sleep.
I had a woman ask me one time: I have never been on a boat, do you think I will get seasick? I asked her if she ever got motion sickness. She told me that if she rolled over in bed too fast, she would become nauseated. I told her I felt that there was a very good chance for her to get seasick. She went fishing anyway. We fished the bay and it was windy and a little choppy but she never got sick. I also have had people tell me that they were old sailors and never got sick, but they only lasted about five minutes out on the Gulf before getting sick.