Seasickness on a cruiseliner is a funny thing. I grew up in Alaska and have spent countless hours in rough water on a sport sized fishing boat in the name if fishing. In my entire life, I can remember getting seasick only one time. Same thing in Cozumel on a cruise this year, we took the ferry to the mainland in extremely rough seas. Half the boat puked, but I was fine. However, you get me on a cruiseship, it is a different story. As soon as I can feel myself moving and I am not able to see the horizon, I get sick. Fortunately, I take a pill or two of motion sickness meds and I am fine.
People have mentioned that an interior room at the waterline is the best. In theory, I have to completely agree and if it is your first cruise, I would recommend it. I have found that in reality, by the time you factor in the size of the ship, the stabilizers, and such, the position of the room relative to seasickness really doesn't matter that much.
So how to find the perfect room not considering seasickness?
Here is the criteria I consider:
1. Interior vs. Exterior: Personally, on vacations, one of my priorities is to get some quality sleep. I tend to favor interior rooms. It is always dark in there and there is no sun to wake you up. To me, the greatest feeling is waking up and realizing it is 9AM. Other people's priorities might be to watch a sunrise over the ocean or to sit and watch the waves with a glass of scotch. You need to decide if you plan on using your room as a place to sleep or as part of the experience. If it is only a place to sleep, don't blow the money on a balcony suite. If you like your own private balcony looking over the water, then by all means. The decision is yours!
2. Location to public areas: I tend to stay away from rooms near noisy public areas. Most cruise ships seems to have designed this with noise in mind. However, I would shy away from rooms near the entrance/exit to noisy areas like bars. There is nothing worse than trying to sleep with drunks banging into the door at 2 AM as they leave the club. I would say the same with staircases and elevators.
3. The running/walking tracks. I have been on ships that have exterior rooms overlooking walking decks and running tracks. I can't imagine waking up to some health nut running by at 5 AM. On the walking tracks, I used to watch people stop and look into the windows.
4. Locations relative to public areas (again): Not knowing your age or physical health, you need to consider where your room is to everything else (like dining room, disembarkation points, etc). As a person in the mid-30's with no real physical issues, it does not bother me to walk the length of the ship to get to the dining room. Stairs are not an issue. If you are older and the knees don't work too well, you might want to think about getting a room on the same floor as the dining room. The only thing that does not seem well planned on any cruise I have been on is the elevator. The elevators always seem slow and crowded.
Good Luck!