We've been in 719 twice and never noticed any vibration. Also, there is a small advantage of being on the starboard side on the standard 7-day cruise (odd numbers) because you may see the dolphins swimming along side one of the legs early in the morning. Not a sure thing though.
As Ngaire said, most of the cruising is done while you are asleep on this itinerary, so it is very unlikely that movement will be a problem.
You will love the cruise, and I think you'll get a lot of use out of your balcony.
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Eugene