Relatively few passengers get sick. First of all, many of today's larger ships are equipped with stabilizers that take some of the motion out of sea travel.
Large ships are less affected by wave motion siply because of their size.
Many smaller vessels do not venture out into rought waters where motion can become a problem. But, if you do feel queasy there are some good ways to deal with it. Since some seasickness remedies have side effects, try the most basic first. If you are feeling seasick, go out on deck and get some fresh air. Eat something light, such as a few crackers, to get something in your stomach.