Sally wanted to get back in the water and she was thankful her parents had a saltwater pool instead of a chlorine one. She went out to the pool deck and took a quick look around. Her parents weren't home and the privacy fence around the backyard meant neighbors couldn't see in. Sally untied her bikini top, which her dorsal fin made uncomfortable anyway, then took off her bikini bottom as well. Then she dove into the water, which felt wonderful. Sally began swimming laps freestyle, which she noticed slowly became easier.
When Sally stopped for a break she brushed her wet hair from her face. Something caught her eye and she looked at her hands closer. They were now gray with a thin webbing between her fingers. She lifted one leg and wiggled her toes. Sure enough her feet had also turned gray and there was webbing between her toes. Sally resumed swimming and was amazed how much more effective her strokes were.
As she continued to swim, Sally's butt began to feel strange. She headed for the steps out of the pool and found she practically had to crawl out. She reached back and scratched her bare, itchy butt. She narrowed the source of the itching down to her tailbone. She scratched at the itch until she felt something begin to grow out of her. It was just a nub at first, but it slowly grew. Sally felt it has it extended and started to take a more defined shape, including a vertical tailfin.
Sally got to her feet, which was awkward with the tail throwing off her center of gravity. She went to her swim bag and got her smartphone which she used to take photos of her tail since it was difficult to see behind her. Her new gray appendage was several feet long and it was definitely a shark's tail.
The shark DNA was clearly having far more of an effect on her than just letting her regrow teeth. She was both excited and a frightened. How far would the changes go? Were they permanent?