On RCI Bonine (Meclizine) is available onboard all ships, from either the medical facility or the guest relations desk, at no charge to guests who are feeling sea sick.
The free with RCI Tablets seem ok if you get an injection that will cost you as the ships doctor is not cheap,some cruise lines send you to the doctor for any treatment,but if your a bad traveler there is a patch you stick behind your ear which you can get on prescription from your own doctor these are very good but may have some side effects?
There are travel accupoint bands you can buy in some supermarkets and pharmacies
i found them very good also crystalised ginger or ginger ale work wonders. it would have to be very rough to feel sick on these big liners. i cannot do ferries
but have had no trouble as such on big liners.