I swear by green apples for motion sickness. Something about the pectin they contain. I usually don't get seasick when on a cruise, yet I do suffer a bit of the mal de mer when on a small boat. It just so happened that I was on a very small boat this past weekend (a 20-footer) and I was absolutely fine, but only after one green apple.
As others have said, it's unlikely that seasickness will interfere with your cruise. And, as far as remedies, everyone's different.