Wow, I wouldn't have guessed that so many people dive with seasickness meds that make them drowsy.
From the non-medical perspective, the Navy has actually done a fair amount of research into ginger. A few people already mentioned ginger, but I think there is more to be said. The Navy found that ginger, (and peppermint to a lesser degree) has a numbing effect in the GI tract. In sufficient quantity his effect is complete, but it only works for 65-70% of the test cases. Also, the studies found no ill effects to ginger in any reasonable quantity. So the good news is it DOES work, but the bad news is it doesn't work for everyone. It is possible to be immune to the effects of ginger.
I have never had any seasickness issues, nor had I ever been airsick before flight training, but aerobatic flight (especially inverted) followed by moderate turbulence never failed to make me puke. Since Navy pilots are absolutely forbidden to take meds that are not prescribed by a flight surgeon, and there are zero Navy approved motion-sickness meds, all that is left are naturalistic methods. Here's what worked for me.
I would eat jasmine rice (I doubt the type matters) with about a teaspoon of grated fresh ginger in it every morning that I would be flying. The effect lasted about 4 to 6 hours.
I personally think fresh ginger is the way to go because it is cheap, you can buy it at your local supermarket, and (I think) it tastes good. It is a bit spicy, so I imagine that not everyone will like it. Using ginger ale as an alternative doesn't really work because there is actually very little ginger in it, it is used only for flavoring and most of the ginger is processed out. Ginger pills definitely work, but I would be extra careful to read what else is in them. Many ginger pills have caffeine, which is counter-productive when trying to avoid motion-sickness.
A few other techniques that were taught to me are cool fresh air blowing on your face, staying away from spicy foods (ginger doesn't count, ironically), and accustomation . At the risk of dragging this post on, I'm going to say something about accustomation. The Navy uses an evil spinning barstool device. It can make anybody puke guaranteed. However, it has to be combined with positive visualization techniques to actually train your brain to accept the stimuli coming in from your vestibular and somatosensory systems. Without these, it is possible to train a person to be sick just by sitting on the *&%# device. I suppose this is like Pavlov's dog stuff, and I'm sure a boat would be no different. You can use it to accomtomize yourself, but only if you can stop it when you need to (maybe by going for a swim), and only if your brain learns the right lessons. Otherwise, you can train yourself to be sick just by stepping on the boat.
Who knew the Navy was in to such mamby-pamby new age crap?
By the way, I am just parroting some information given to me by a flight physiologist, who is a specialist in hypobaric (not hyper) physiology. If anyone here can add to this or provide some citations for any of this, I would be grateful.