My fishing buddy would
always get seasick. It got to the point where the other guys in the fishing club (me too) would bet on what time he'd start to hurl

After about 20 odd years of seasickness, it no longer affects him.
My dad found an Ann Landers/Dear Abbey article that said to tape an aspirin in your belly button. He tried it and it worked.
Mind over matter?
Then again we don't close the bar the night before a trip anymore either.
What I've learned over some 25 or so years around the water...
Do eat a good breakfast.
Don't eat a greasy breakfast.
Do stay outside on the boat.
Don't stay in the cabin. Ebbtide's theory: Your inner ears sez that you're moving, but your eyes say that your not, because everything is moving along with you. You need the horizon to act as a level)
Don't stare at your line in the water (when fishing)
Eat salty treats during the day, like Ritz, Saltines, pretzels etc.
Have plenty of water or soda. (Drink beer? Sure but you may undo all of the good that you've done already)
Stay away from the exhaust from the boat.
And if you do get sick...
1. Heave over the side. Don't mess up the head for the others. Its OK to hurl next to me...fish like chum

2. Stay out of the cabin! That will only make it worse!!! Find a nice place to stretch out on deck (or curl up in a little ball if need be).
3. Eat some crackers and drink some water. Even if it is something to toss up later, cause (they tell me) it beats the dry heaves.
And lastly if you have the ghoulish need to push someone over the edge into seasickness (like for instance you're'bet' time is coming) a little green food dye in the tuna salad does wonders :evil grin: