There are three types of boaters.

Those who admit to occasional sea sickness, those who never venture off a mill
pond, and those who tend to "forget" the last time they were seasick.

Some people are more prone than others.

General cause is a mismatch between the
sensations your body is feeling and the visual input to your brain. If it's
reasonably rough and rocky and I try to read, for instance, I can begin to feel
some effects.

For many people, keeping an eye on the horizon and getting plenty of fresh air
are
important. Ginger, either in a root form or even in ginger ale tends to help
many folks. Others will require Dramamine or other medication, and some small
percentage of people probably just don't belong on lumpy water at all.

Obviously, the rougher it is the tougher time your body will have reconciling
what it is feeling with what it is seeing and you will be at greater risk.