Who gets it?
Everybody. 90% of all people will have suffered from motion sickness at some point in their lives. Almost anyone who has normal inner ear balance function can suffer motion sickness - even sailors.
Sleepiness can often be the first sign and some people who think they don't get seasick actually do without realizing it. People who love to take a nap the moment they get out onto the water are probably feeling the effects of mild motion sickness.
After sleepiness comes the nausea but it is often mild and may not be much of a problem. Studies have shown that maintaining a positive state of mind can help offset the effects.
For many unfortunate souls the symptoms escalate to extreme nausea, vomiting, dizziness, headache, pallor and cold perspiration.
If you have a problem with motion sickness in automobiles and airplanes, you may be more prone to seasickness. On the other hand, if you get nauseous in a smallish sailboat, that doesn't necessarily mean you'll get seasick on a cruise ship.
Review the precautions discussed on this site and you may reduce the severity of seasickness.