I think one of the things that really matters is the size of the ship as well. My wife and I went on the Carnival Conquest and the Valor which as some know is basically the same ship with a different decor. We didn't feel the ship move that much on either one of those cruises. Now we just got back from a RCI cruise on the Jewel of the Seas and this ship is about 30,000 tons less than those two Carnival ships and the first two days it was fairly rocky. The wind was around 24 to 30 mph and the seas were about 9 feet or so. We could feel it that's for sure!
Royal Caribbean Jewel of the Seas-2007