The various natural remedies are electronic wristbands; equalising sunglasses; ginger sweets; homoeopathic tablets (they don’t work), fixing your gaze on the nearest point of land; lie down on your back in a comfortable shaded position, close your eyes and relax. Go for a swim.
If you are feeling sea-sick then get yourself to the lower side of the boat as quickly as you can. This will avoid getting vomit blown all over yourself and other guests.