We sailed on Imagination's sister ship Ecstasy and had an oceanview room that was GREAT! Choose something midship...we had R143 which was a nice central location, nice smooth ride, close to middle elevators but not noisy at all. The only improvement would be to get higher up - unless the motion would bother you. The higher your deck the more you feel the movement of the boat. We noticed the movement in our friends/family cabins on the Empress deck and in the dining room but it did not bother us so we would go higher if upgraded. We probably wouldn't pay more to get a higher deck though, since the Riviera was just fine. We had absolutely no complaints about our cabin
Another thing to keep in mind is to not get a cabin right below the galley or lounge due to possible noise nuisance. I think being below the dining rooms and shops would be okay. And sometimes too close to an elevator can be noisy from what I've heard, but not the case with our cabin. A few other posters have said that the noise from elevator areas is blocked well and shouldn't be a problem on any Carnival ship.
I hope this helps and I WISH I was sailing on or near the 7th, but must wait until next September
Have an awesome cruise