I read over and over, 'the ship is so big, you don't feel it rocking.' Well, if there are high seas, you SURE do. Also, if you are motion sensitive, you will feel it even on calm day and in port although it probably won't make you sick then.
On our 10/24/04 cruise, the crew reported 14-16 foot swells. It was awful for me. I was SICK all night. It started about 5:30 PM. I could not stay at Dinner in Animators. Dh tried to stay with our sons and he said that the dining room was practically empty half way through dinner.
When walking through the corridors, one would dump the wall on one side and two steps later, bump the wall on the other side. For those not prone to motion sickness, it was sort of adventurous and comical. Those people could go on with regular activities, although I would not let my kids on the outside decks. My 3 year old son was sitting on the top bunk in our room and was thrown off. For the people with motion sickness, it was a BAD night.
(I do think this was somewhat extreme and unusual. The Captain finally had to make an announcement that 'the ship is not in trouble' because there were a lot of unsettled passengers.)