A yacht
broker I've worked with (Capt'n Bob Sherman) turned me on to a drug called Promethazine (or Phenergan). It is used by NASA for shuttle astronauts as a high percentage of them suffer motion sickness during launch. While they inject it into their thigh as they need it, it's also available in pill form and is used to treat chemo-related nausea and even morning sickness.
It requires a prescription and after my Dr. read up on it, he said give it a try. This is the greatest stuff since sliced bread. It completely cures motion sickness even well after onset. Neither me nor any of my guests have experienced any side effects including drowsiness or any ill effects from having a toddy or two. (Yes, I've closely monitored my guests since the Rx was not issued for them.)
This is far superior to wrist bands, Bonine, ginger, etc. Highly recommended.
Also, a more readily available aid is
water. Dehydration is a contributor to motion sickness and drinking all the
water you can hold will help keep motion sickness at bay.