"Our sea turtles both sustained injuries when they were in the wild that affects their buoyancy. To help them swim, they have specialized weight packs on their backs." -from the employee I asked, and their website.
She was bitten by a shark before she came to the aquarium. She was rescued in Florida and the black thing is a weight to help her swim.
Bubble butt syndrome...it's a thing, Google it, (because the internet never lies, and card catalogues don't exist anymore) the weights are therapeutic. Snag someone next time your there and they will give you the DL on the treatment.
I've been told it was a tracking device
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