We have traveled on a Catamaran a few times with people prone to motion sickness. I suggest two things --- one pack Dramamine. It's best to take it when you are anticipating the motion sickness. The second thing is as soon as the boat starts moving come upstairs and do not stay in the cabin. The great thing about catamarans is they are more balanced than a monohull so that makes a big difference. Being in the open air can sometimes be the best medicine!