Your physician is the best person to answer these questions.
I can tell you one thing from personal experience that helped my wife. She tried a dose of the medication before the cruise to see what effect it had on her. It was better to know before the cruise than to ruin it while she was on it. This was on the advice of her doctor.
All of the OTC motion sickness medications are preventatives and offer little relief once true motion sickness sets in. Ginger does help the sick feeling you do get with motion sickness.