For example, suppose you have guests on your boat, and you are suddenly hit with a major bout of gastro-intestinal distress. You run to the head, do your business, and soon the whole boat knows it, because they can smell it.
Or suppose that you have your kids aboard, and you wish to get amorous with your wife. Totally out of the question.
Living on a boat, to me, isn't as romantic as it sounds. It seems like a good idea if you are a loner. If you need company, it is a yacht or nothing.