There are organs in your inner ears which detect whether your head is vertical or tilted. Without them you would fall over.
(They are called the "semi-circular canals.") If you rock your head back and forth many times the detection is more than your
brain was made to receive and you get sick. That is what happens to you when you get seasick on a rocking boat or motion
sick in a car.
Prevention and Cure
Both prevention and cure is to stop your head from rocking. When the boat rocks to the left, you rock your head (or
your whole body) to the right so your head remains vertical. If your head remains vertical you will not get sick.
The boat swings left and right ("yaw"), rolls side to side ("roll"), and tilts forward and back ("pitch"). No matter. Whichever way
the boat goes, you go the other way the same amount so your head stays still.
The boat also rises and falls, like an elevator. You may not like this motion any more than in a fast elevator, but elevator
motion does not tilt your head and does not make you sick.