Hey xjennybeanx - I agree with the others - try not to overthink it. But when our daughters took their first cruise with us, I bought this great stuff at Wal Mart for $4 or so. It's called Motionease. It won't make you tired. We hit a rough patch one night on our cruise and they both tried this stuff and it really seemed to help them. You just put a drop behind your ear (like when dabbing on perfume) and that's it. Go do what you want. I didn't know if we'd need it or not but I figure, just in case the mind wins the argument with the body....it's cheap enough... why not just have it along anyway? Nobody's fun got ruined and they had a great time.
Just a suggestion but yes, like the others said.... definitely don't get them thinking about it. Just have a great time and enjoy your vacation with your girls. A navy guy once told me, if you feel a little sick....keep moving and get some air and go up higher on the ship. The worst thing you can do is lay down. I had a coworker that went on a cruise and someone in their big group laid down and never got up the rest of the trip....spent the whole time in bed. What a bummer.
Hope that helps a little.
Have a great cruise!