Looking at the horizon line is effective, its about your eyes and ears agreeing on the information they are receiving. If your ears are telling you that your moving but your eyes only see the static area around you (inside the boat, close water, etc) then your eyes are saying 'no movement here buddy' and then your brain gets confused and you feel sick as a result. Looking at the horizon (or land if your are within eyesight) gives your brain a visual reference that ties into the inner ear sensor input