She is a sea dragon {sea serpent}, who can change into human form when she wishes, she's also got pirate in her, she was in the past the pet of Davy Jones and was huge and every ship that sailed suffered in her wake.

One day Davy Jones tires of her as his pet and releases her bond from the dutchman & tells her that if he ever saw her again he would kill her and he gets his new pet the kraken.
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She then fashioned herself into human form and swam to Tortuga and came across Captain Jack Sparrow and they got to know each other, he told her stories of his life and she told her story and since they both had a bone to pick with Jones they became firm friends.

Aquaria was there during the time that Jack was captain of the black pearl and stayed with him through thick and thin even when Barbossa left Jack behind on that island.