The inner ear, where your sense of balance is located, gets out of sync with your visual input. If they can stay in sync there will usually be no problem
Watching the horizon, anticipating the motion caused by oncoming swells and letting yourself be part of the movement of the vessel are the keys to avoiding sea sickness.
When I have had people on my boat that were prone to illness, I would put them on the tiller to steer. It forced them to watch the seas and anticipate the movements. It always seemed to work.
One other factor, people who worry about getting sick, get sick. Those with a positive attitude and are part of the whole picture do just fine.
It's strange but I never liked to ride with my father because his driving made me a little sea sick. I never figured it out. Nothing and no one else ever affected me that way.
I don't really get sick if I am riding a boat, maybe a tab bit dizzy. If I swim all day my legs get tired
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