Oh, I'm never, ever sick at sea.

Yeah, right. Under some conditions anyone can get seasick. Some people are very immune, but one day it will happen even to the most resistant boater. Mal de mer ranges from mild discomfort (green around the gills) to a medical emergency (serious dehydration from vomiting). You don’t want to become so incapacitated by seasickness that you can’t function and thus pose a danger to yourself and others aboard. There are lots of articles and books that discuss seasickness prevention – the best method is the one that works for you. Try several to find out. Sitting under a tree works for everyone. See pp. 361-362 in the Mariner’s Guide for a discussion of seasickness.