As others have said, theorectically, the best location would be midship to minimize any possible up and down motion. The best location to minimize left to right motion would be lower in the ship, such as deck 2. So the best location would then be deck 1 or 2 and midship.
However, if you want the ability to step outside on a veranda, the lowest deck would be deck 5, but they are only in the aft section of that deck. That leaves Deck 6 midship as the best location for a veranda cabin, for avoiding any potential motion.
We have been on decks 5, 6 and 8 and always try for as close to midship as possible. Our adult DD was in an adjoining cabin on deck 6 with her family last year and she did have problems with sea sickness, but we thought it was a pretty smooth ride. My DW and I have never been bothered by motion sickness on any of our five
DCL cruises.
Again, as others have stated, on the first evening when the ship passes over the Gulf Stream, you are likely to notice a little more turbulence. It's inconvenient because it's also during the first couple hours of the cruise when people are adjusting to being on a ship, some for the first time. It's usually over by 10 or 11pm at the latest.
The Disney ships seem to have very effective stabilizers and usually minimize the turbulence on the water for most people. I was on one of the Carnival Fun Ships a couple years ago and with the wave heights reported to be approximately the same size, we noticed a lot more motion than when we were on the DCL ships, and on all of the decks, but especially the higher ones.
I would recommend that anyone that was susceptible to motion sickness take Bonine daily as a precautionary measure. If you would be more comfortable, you could try using half of a dose instead of a full one. I would also recommend that you try using it a week or two before the cruise so that you know if it causes you to experience any side effects, before you have to depend on it onboard.
You could also try using Ginger Root capsules or an inexpensive pair of the wrist bands if you want an alternative to Bonine.
A cruise is relatively expensive for most of us and something we can't experience every week. For people like me that are susceptible to motion sickness, I feel that a little preparation to protect an enjoyable time is worth the trouble.
hope that helps