Now for the serious answer.. Water is usually calm in the area the cruise travels. There is a fairly strong current and Steve and the crew will cancel or postpone the cruise in event of rough seas
I tend to disagree to a point. Often the sea has large swells and water will splash over the front of the boat. So if you are worried about motion sickness, I would recommend some preventative care.
If you're concerned, take some Dramamine and just relax. We've never had a problem largely because the party keeps you distracted.
We are a little worried about this as well, she gets motion sickness and when she takes something it can sometimes make it worse.