I will start with saying many people will not believe this. Sea sick is like allergy, is psychological, or has a large psychological component.
I had trouble with the rocking motion in the beginning. I say to myself I am not going to get sea sick, and visualize myself being perfectly happy with the rolling motion. If you are insistent on you being fine, then you will be fine.
Is the same thing with allergy. I use to have runny nose from spring to late summer. I say allergen is my friend. I will breath it in. Have not had allergy in last 5 years.
Finally, the common cold and flu are psychological as well. You will get sick if you let yourself. Last time I came down with a bad cold was probably over 10 years ago.
The psychological component in health is well documented as the placebo effect. They account for that in every clinical drug trial. Happens that people keep buying medicine and ignore this powerful potential Drug companies are perfectly happy with that. You can fix any issue with medicine. But is better to let your body handle it on its own as much as it can. You will be amazed how much your body can do.