My mom gets motion sickness a lot, but we cruise all of the time, and it rarely bothers her onboard. However, if we have an excursion with a long boat ride....
The ship is so big, that it doesn't rock much. I don't even notice it. My mom does have a night or two where she has to take a lot of dramamine, but no terrible motion sickness.
However, I do think it depends on when and where you're cruising. Cruising the Caribbean has been a little more rough than cruising Alaska, where the water seems very still.