I looked into this when I first started boat fishing, I got really sick in a certain kind of swell and didn't like it at all

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It's all to do with the eyes telling the brain you are stood still and the balance in your ears telling the brain you are moving that makes you feel sick , so if you can get them to agree with each other you will be fine, that's why you should always watch the sea , not the inside of the cabin or the boat deck, they appear to be motionless to the eye whilst the ear is telling you they are moving.
Ginger definitely helps , root ginger is a bit harsh but by far the best, chrystalised ginger I could eat all day anyway (they put it out on plates on the cruise ships when it's rough) or ginger biscuits will help, but all of these just help your stomach ,not your brain/ears/eyes.
The best way to avoid the sickly feeling is to concentrate on your fishing and try not to take your eyes off the sea , sitting looking at the deck whilst tying on a new rig is the quickest way to sickness.
After several trips out you will eventually get more used to the motion and feel sick less and less , it's far easier with your own boat to overcome sea sickness because you know you can choose to come back in whenever you want , when someone says it's all in your mind , the biggest part that's in your mind on a charter boat is knowing that you are stuck out there and can't come back if you are sick, that alone is enough to make you feel sick before you get on the boat

It's a horrible feeling, but one you can definitely overcome with practice
