Go to motionsicknessstore.com. They sell Bonine and offer advice on when, how often to take it. They also have some other medications. I have always been advised to take Bonine or Dramamine as well as Ginsing tablets. The good thing about Bonine is that it comes in chewable tablets, so you dont need to have water or something to swallow it and it also apparently gets into your system faster.
I would certainly buy it before you get on the ship as I am sure they raise the prices tremendously once you are onboard.
Hope it works for you!