Watch what you eat and avoid greasy foods during the entire trip. Eat moderately but avoid having an empty stomach. If you feel that your stomach is starting to rumble, munch on a piece of cracker or toasted bread. The starch on these foods can help settle your upset stomach and prevent you from throwing up.
Alcohol and coffee are also big no-no. Wait till you are back on solid ground before indulging in them again, but while on cruise avoid them like the plague. Both are diuretics that will flush out liquids from your body, and being dehydrated will lower your resistance against seasickness. Be contented with sipping water and juices if you want to enjoy your trip.