Each person reacts differently, so go prepared. There are plenty of over the counter meds, but be sure to check for her age group. I have cruised in July on the Conquest, and the waves were at times a little rough, but she probably will do better than an adult.
They always seem to surprise us, don't they? Does she get car sick? That is a lot of times a good indicator. Remember lots of fresh air is good for her.
Good Luck on your cruise. It will be a wonderful experience for all of you.
P.S. I hope the warning from the poster above was kidding about the alcohol. I think we can safely say that your 6 year old will not be served booze.