OP asked about spot most suitable for motion sickness as well. Outside balconies are not the best option for this . The closer to the center of the ship and lower the deck the better .
I personally would go with CP deck 9 , that way if there were times when this became an issue , at least you can come out for air , but still not be facing ocean .
The other option as I think Biker stated is a very low inside cabin . Definitely the least movement you can possibly find , but , If you ever were to find yourself not feeling so great , you are confined to a very small space with no open air .
Most likely either way you would be fine as these Megaships are extremely stable on the water
Cheers