I can't realy speak for getting sea sick. Never have. Even running into that nasty storm coming back from the MED in'92. Walking on the walls, yes, Sea Sick, no. The crew members who were sensitive to sea sickness used patches that the Corpsman on the ship provided and seemed to do fine standing watch at sea. That being said, that ship was a US Warship and a whole heck of alot smaller than the cruisers at sea. The first Cruise we took was on the MS Imagination (Carnival) in the Carribean in May which is realy calm. The shipwas in motion but you could hardly tell that it was. Most people never noticed it. My wife didn't notice it at all. I could tell that the ship was moving but just bearly and only because Im used to being at sea.
I would recomment patches and use them if you realy feel uncomfortable. I tend to suspect you won't even notice the ships movement.
Enjoy your trip