Seasickness is not caused by motion: it's caused by a sensory conflict between the inner ear, close-focus vision and far-focus vision.

The proof of this can be found by any person trying to read while riding smoothly on a straight-away in a luxury car. Not much motion to speak of, but plenty of sensory conflict between the close-focus vision trying to concentrate on the book being read, and the far-focus vision peripherally picking up scenery moving outside of the car.
The only way to cure seasickness is to get all three senses lined up. On my boat, whenever someone appears to slowly be turning green, I put that poor soul behind the wheel of the boat, point to some landmark far away on land, and tell them to steer towards it for the next half hour.
That action brings all three senses into line and removes the conflict. It also takes away the fear of loss of control that most people new to boating feel when coming aboard.
Some people seem to want to get seasick: that's all I can conclude by their actions prior to coming aboard for an afternoon sail. Here's how they accomplish it: eat a lot of greasy food, drink alcohol, smoke, and do not bring any items of clothing that will keep you warm in an offshore ocean breeze.
Here's a tip to boat owners: if you happen to notice that a guest seems hesitant to make that giant leap of several inches from the dock step onto the boat - without a net - grab the guest's hand and see if there's a Kleenex crunched in there.
Those two signs will be an indication that you're going to have a green person in a little while.

Drinks LOTS of water before a risk of seasickness develops.
Get lots of rest.
Don't eat greasy foods.
Drink MORE water.......if you get sick it won't be as horrible.
MORE water......
Take lots of fresh ginger which should be carried on any boat.
MORE water.

And don't fight it.
If you're going to get sick, vomit soon and often.
You'll feel better than trying to hold it, which won't work, anyway.

Some new gadgets work a little at redirecting attention from the visual sensing of far and close objects.  The watch-like object has been shown to direct attention to this constant shock in the left arm and helps relieve seasickness somewhat.  Typically after 3-7 days at sea most all people stop the seasickness or they die from it after throwing themselves over board (never had that happen).