So do I, but hardly ever on a cruise ship, especially the larger ones. To be on the safe side I take Stugeron for the first couple of days. I also have battery operated "seabands" just in cas.
For sea sickness I highly recommend Travel Bands which prove very effective without causing drowsiness or other side effects.
Yes, we also thought we might be sea sick on a cruise, this is why we had never cruised until 3 years ago, but we have now been on 5 cruises (3 to Med, 2 to caribbean) and think it is a fantastic holiday, and have never suffered any sickness at all, but we did purchase the wrist bands from Boots and pills just in case, as I believe the injection that you can get on board is expensive, we did hear of people feeling a litle queezy, but this has not happended to us at all. We have ever experienced rough seas on the Med an we went in Sept/Oct time, and there is hardly any movement at night whilst in your cabin bed.
Have to be honest and say that we did a Med cruise in September and there were some very rough nights but obviously that will not always be the case, but please be prepared. Reception do supply sea sickness pills and the ship's doctor will prescribe an injection. If you still want to go ahead with the cruise, can I suggest Stugeron which you can buy from the chemist.. they really are very good.