An age old remedy is at the onset of symptoms, to wrap your arms tightly around an oak tree.
There are two types of sea seasickness.
Type 1: "Oh my God, I'm gonna die."
Type 2: "Oh my God I'm not gonna die."
Really though, Bonine has a fairly good success rate, but everybody is different. You may also want to try Triptone, also an over the counter remedy. With either of these, it seems to help to take a dose the night before when you go to bed, and then again in the morning about an hour before the trip. For hard cases Scopalamine patches seem to work well, but you'll need a scrip. There's also a remedy that the sailboat racers seem to think is best, but I don't recall the name of it. Anyway, whatever it is called, I think it's not available in the US, you have to get a scrip in Canada or something and then bring it back.
If you can learn to gimbal your head with the rest of your body and allow the boat to move underneath you, you may become less prone, but this takes practice.