I was in a stateroom over New Year's. Didn't notice any ship motion. If you're getting a suite, the deck shouldn't matter, but if you are getting a regular stateroom, you need to look at the deck plans and make sure you don't get a room next to the center (where there's a big blank space) - that's the intake/vent for their garbage recycling - you get a sicky sweet smell of rotting garbage and the noise from the vents is very loud. That's the case with both Emerald and Dolphin decks. The actual space of the balcony was great and again, we didn't notice any more motion than elsewhere on the ship.
I was in C750 (a PS Suite) on the Star Princess (layout is very similar to the Diamond) a few years ago. If anything, the sound of the prop wash was actually very soothing. My only warning about the aft suites is to be careful of which one you choose. C750 and it's sister on the starboard side C755 have part of the ships outer structure that blocks the view from the glass doors from the bedroom portion of the suite. It also take up some of the room on the balcony so the two lounge chairs, two regular chairs and two small tables can be a little crowded.
If I were to choose these suites again, I would go with C752 or C757.
In the attached webcam of the Star...Caribe is the second deck of suites. If you look in the middle of Caribe you can see both sets of patio doors for the interior suites (752/757) but when you look at the outer suites (750/755) you can only see one since the ships structure blocks the view.
We've had two aft balconies on the Star, and loved them. Both of our aft balconies on the Dolphin and Emerald decks were adjacent to the air filters mentioned above, and we didn't notice a thing. In fact, we loved the cabins because they are so much more private with only one neighbor. We also noticed no motion.
The only thing you might want to know is that you don't hear the waves against the side of the ship as you do in a side balcony cabin. You hear the whooshing sound of the wake, which is quite different than waves hitting the ship. We love the sound of moving water.