Deckhands and I used to place wagers on who would get sick on a trip. It was rather easy money. IMO 90% of seasickness is caused by one of three things; Fear, Lack of Sleep (up late drinking night before) and for some weird reason being of African heritage. The other 10% comprises those who have inner-ear issues or regularly fight nausea.
For those 90%, the best thing I have seen is Orange juice. Something about the acid in orange juice is darn near a magic drug. The moment you feel your stomach start to go sour drink a small bottle. Take quite a bit as it usually will need to be done multiple times during a standard fishing trip.
Other things that help are watching the horizon, getting plenty of sleep, avoid alcohol at least 12 hours before trip, and staying the hell out of the cabin. And whatever you do...if you are feeling sick don't go into the head, go to the rail or grab a puke bucket!
And if you are African-American...sorry, but I suggest you skip the charter boat trip. I've personally taken somewhere between 40 and 50 clients of color out on 4 to 8 hour trips. I have yet to meet one that hasn't gotten sick. Friends of mine in the Navy agree and tell me most the black sailors take 3-5 days longer to acclimate and get their sea legs and bellies stable. I have asked a good number of doctors why and have never gotten a solid answer other than African-American males do seem to have a higher occurrence of motion sickness than do Caucasian and Hispanic patients.
For that other 10%...Go see your doctor. Pills and patches that help with nausea or vertigo do help. But most over the counter "take my best shot" stuff rarely does. And take orange juice with you also unless your doctor says not to.
The other thing I might mention here is what to do once you get off the boat. If you are not on water every day the ground will be moving on you even after you hit the dock. Find a place you can take your shoes or boots off and walk around bare foot or in your socks. Also, find a place to get a milk shake as soon as possible. You'll be amazed at how much faster you will recover getting a chocolate or vanilla milkshake down right away.