You actually don't even have to buy the Bonine brand. They have the generics at Wal-Mart. The active ingredient to look for is Meclizine.
I've only had to use it a few times during rough seas, and it does work. Most people I've spoken with that are prone to motion sickness recommend taking it before hand.
The normal movement of the ship doesn't bother me very much. As a precaution I've been taking the Ginger pills/supplements that Wooddon suggested in the prior post as a "just in case" measure the last 2 cruises. They really do seem to help prevent that feeling from even starting with me. I usually start taking them a few days before the cruise.