Your neck and back helps tell you that you are leaning to the side. Your feet are important, tells you if you are on an incline. All these are tied together in the brain. If you can't see clearly, or are in a fun house or given an illusion, you may feel unsteady. New bifocal glasses can also produce a similar effect.
Children generally get car sick, as do dogs.
After a while, however, they learn to compensate and no longer have this problem. When I teach in China, the first question I am asked is why the native Chinese get train sick and the foreign Chinese from Taiwan, Hong Kong, and USA do not. It is simply that the foreign Chinese have been raised with automobiles and have accommodated and the natives have not had this experience to learn accommodation. If you get elevator sick, turn your head up so that the elevator motion affects the ear canals having to so with driving, which you are accommodated for. Turn your head so the boat motion is like an automobile stop and go motion.