J.G.
I think I have heard that the ship is big enough that sea sickness should not be an issue except for the extremely sensitive.
Call the cruise line you are going on and ask about what they do to accommodate toddlers. Activities, child care and so on, so you can be prepared ahead of time and make plans. Bring some small familiar items of hers as well - favorite blanket stuffed animal. And then pack some small surprises for moments when you might need them.. a small coloring book and new set of crayons, a board book or pop up book, a small new toy. Something to hold her attention and give her something to look forward to. Will she be the only child in the family going on this trips.. cousins?? see what you can do to have them interact.. if not ... see if there is a way to meet other parents with small children on the ship.
it will just take some planning ahead and learning everything that will be available to you on the cruise.