Great questions....I've been on Sensation and it's a nice, fun boat!!
When you go on an excursion on a boat, there is ALWAYS someone from the crew off the boat to watch your stuff. In Nassau, there are few water excursions. Just take a taxi to ATLANTIS. Everyone deserves to see Atlantis once in their life. If you're going to Freeport, can't give you any advice. Haven't been there for years. But your belongings are usually safe.
As for boat rockin', the closer you can get to the middle of the boat and the lower on the ship the better you will be. HOWEVER, if the boats a rockin, it doesn't really matter where you are!! For the most part, it isn't that bad!! As for boat motor noise, don't worry about it!! People like to complain about every little thing. I've been on the back and on the bottom and slept like a baby!! The boat noise isn't that bad!!
Have a great cruise!! And try to chilled Cream of Zuchini soup!! It's "to die for". My opinion.