The Georgia marine aquarium is absolutely amazing. It even had beluga whales the last time I was there, although that was a few years ago. They were trying to breed them, but not having much success. It was still incredible to be able to see them so close through the glass.
They also had manatees, which were also a conservation effort.
I have to say though that the Monterey Bay aquarium is my favorite, simply because they have the best selection of jellyfish. Blue lit jellyfish are one of the most beautiful things in the world to see, and I fully recommend this aquarium (considered one of the best in the world) to everyone.