I will share my DH's experience with you - he gets motion sickness easily if not driving a car, can't do any ride that goes in circles, no Body Wars etc. He always takes Bonine before he flies and unless it is a bad flight is usually OK.
We cruised 10/7-14/00 without our children and he was FINE. He took a Bonine every morning starting Saturday just before we left for port and didn't have any problems. He ate, swam, drank, spent time on deck and was never bothered. We also cruised on the Magic Christmas week last year and I will say the seas were calmer that cruise but I also think it is a day to day (or wave to wave) experience - one can't count on Mother Nature.