I was really concerned about DD3 (at the time) and seasickness because she tends to get car sick. You know what, she didn't even feel the motion of the ship, kept asking when the boat was going to move.
My pediatrician recommended Benadryl. We brought a whole medicine cabinet full of things, including Amoxinillin in case anyone got sick, and didn't use anything other than Advil for Mom and Dad.
Have a great time!
Denae