I don't know how well this will work on cruise ships, but for other motion sickness, I've taken Dramamine, the less drowsy formula. That you can take one hour before starting travel, or in this case, an hour before you set sail.
I actually took it for my first cruise, not knowing what to expect, and I'm glad I did, because that first night there were some really choppy seas, and the boat was rocking pretty heavily. At the Captain's reception the next night, he half jokingly asked if we felt any motion the night before, and we all shouted "YES!" He promised it would get smoother, which it did. Supposedly at the time, one staff member said it was the worst it's been in 6 months. This was back in October of 2007.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas