If you are still worried, invest in a pair of sea bands from Boots. They only cost about £7.50 and are worn round the wrists. They have solved my sea sickness problems completely and are washable, so can be used again and again. Happy cruising!
Unless your cruising through the hurricane season the movement on cruise ships is imperceivable. Your going on a floating hotel not a Atlantic fishing trawler so put seasickness out of your mind. You also have the deck levels wrong, as the higher the deck the more movement you experience. The lower the deck the less movement you encounter. Enjoy it.