Wait they/she didn't plan to end at the ferris wheel anyways?
Maybe tickets for it are too expensive for them, or it's so popular they're hard to come by?
That being said, the huge cruise ships aren't actually shaking around that much, unless the waves get really stormy. She might be totally fine.
Even the smaller boats for things like that have decent stabilizers, plus they wouldn't be going way out to sea. I have serious problems with motion sickness, but didn't experience any on the boat across the English Channel back when I visited Europe.
I also have a fear of falling so strong that even a lot of Ferris wheels give me trouble. It takes a few rotations before my brain realizes that I'm not
actually falling. The larger wheels that rotate slowly don't give me trouble, so I like them a lot better. Needless to say, I can't handle rollercoasters. Abject terror != fun.