Here's the design of Aqua's pre-regal form before she finds a necklace with a pearl that permanently evolved her into her regal form - Regal Pearl.

When Aqua turned 18 years old, she found an underwater box in the mysterious cave with colorful bioluminescence reef and took it to the scientist of Atlantis. He said that gem has the power to make any mermaids have a regal look with big breasts and wider hips which makes them majestic and beautiful.

Aqua's mother, Athena, has worn the same necklace when she was her age after her marriage with her husband, King Samudra.

And after Aqua worn the necklace, her body had changed and her chest and hips grew. After that, Aqua had become a regal look that fits as more than a Mermaid Princess; It fits as the Mermaid Queen of Atlantis.

I hope you like her pre-regal form.