Ginger! Yep that stuff they serve you with sushi. Ancient mariners used it, modern mariners use it. Many divers use it.  You can eat it raw, buy ginger candy, dried, or in capsules.  Available in some natural grocery stores and of course your favorite dive shop - Coral Key Scuba & Travel! 
The popular TV show, Mythbusters featured ginger in their motion sickness episode. Trying all types of motion sickness remedies from prescription drugs, over the counter medicines to natural and holistic methods.  The only remedy that worked on every person was the ginger.

I first learned about ginger from the cooks on my Navy ship. During hurricanes, they would put sliced ginger on the chow line. I don't get sea sick, but I take it with me on every dive trip to share with those that do. I find the best way to take it is the capsules. Some people don't like the spicy taste so the capsules are more convenient.  The candy at the health food stores is convenient, but you have to eat quite a bit to get the effective amount in your system.

In my experience, those that have tried the ginger and it didn't help them took too little amount. It is a food and not a drug. Unless you were allergic to ginger, you couldn't over dose on it.  Every person has a little different affect. Some only need 2 capsules, some need 4 and some need 6 or 8.  The Box we sell comes with 36 Capsules, (plenty for a week long trip). It says 4 capsules at least 1 hour before you board the boat. For some reason if you take it after you are already sick it has no affect. 

Many people that have tried sooo many other remedies report that this is the one that works best with no side effects.  A Florida instructor came with us on a trip. He stated that for 20+ years he was sea sick on every excursion. He just threw up at the beginning of the boat ride and got it over with. This was a minimum of 1 excursion a week as he was teaching.  I shared the ginger with him and his comment to me was, " I've tried everything". Not ginger apparently because after giving him 6 capsules at breakfast he absolutely was amazed that he didn't get sick. He was a believer and said it was the first time in over 20 years that he didn't. 

Hope this helps!