We become sea sick because our balancing organs, the labyrinthine portions of the inner ear, are disturbed by out of level movements, by sudden turning movements, and by sudden changes in movements in a straight line, either horizontal or vertical.
The three semicircular canals, filled with fluid are set on different planes in the ear. When sudden movements occur, each canal is affected differently. The nerve endings have no time to convey information to the brain so giddiness is likely to occur.
Nowadays, seasickness comes under the general heading of motion sickness, a name invented by Sir Frederick Banting in 1939, which includes the discomfort people feel while traveling in all kinds of vehicles.
Sea sickness may vary with individuals from slight uneasiness to complete transition. The symptoms are pallor, cold sweating, nausea and vomiting. People who have lost their ear labyrinths because of disease do not become sea sick. Others become resistant to it. We say they develop their "sea legs", but it would appear to be an adjustment of the central nervous system rather than the organs of balance. Some people find it helpful to keep their gaze firmly fixed on a steady object.