My daughter gets terribly car sick - all the times. She gets sick on the plane and on the rides too. So if we are in a car for more then an hour straight, I give her Dramamine. Most of times it works for her real well.
She also takes Dramamine in parks, as she really likes the most rough and scary rides, but they make her sick. And she takes Dramamine on the plane.
When we first time went for the cruise (and it was the only time for my daughter so far), - I calculated how much Dramamine every person would need for the length of the cruise, and took it with us - just in case.
But none of us had any problems. I was amazed that even my daughter had no sickness at all.
At all. And she starts complaining about nausea after being in a car for more then 5 minutes.
We might have been just lucky with the cruise, as the weather was good all the time, and it was only three days. But nobody used any Dramamine

You just never know.
So don't get worried in advance. Try to think that you are going to be fine, but just take Dramamine with you. And you might get lucky, and have no problems.