"You know what I wish?" Ava said.
"What?" you ask, thinking she might be about to admit an attraction to you.
"Every time I go swimming in the ocean it makes me wish I was a mermaid."
"Interesting. What if I told you I could make that wish come true?"
Ava laughed. "Mermaids aren't real and neither is the magic to turn someone into one."
"I'm not so sure about that. What would you say if I told you I had been given a magic ring earlier today?"
"Not likely. You better prove it."
You think about mentioning making her clothes disappear earlier then think about making her bikini disappear, but come up with a safer idea.
"I wish your purple bikini would turn pink."
Ava looked down and gasped as her bikini changed colors before her eyes. She broke out of your grasp and turned to face you. "You're serious!"
"I am. Do you want to become a mermaid?"
"Absolutely. Give me a purple tail, like my bathing suit was until a moment ago."
"I wish Ava's bikini returned to its previous color then she transformed into a mermaid with a tail the same color as that bikini."
"I can feel my legs changing!" Ava exclaimed. Her legs began to merge together as the material of her bikini bottom began to expand downward, slowly enveloping her legs and binding them together. Ava's wore a huge grin on her face as she looked down and watched the transformation under the water. Then she started to sink for a moment before her bed bobbed up again and she treaded water with her arms.
"Everything alright?" you ask.
"Everything is amazing! My legs just stopped supporting me, but I suppose a mermaid can't stand too well on her tail." After a moment she added "I think it's done."
Floating on her back, Ava brought her new purple tail to the surface so you could both see it clearly. The swimsuit fabric had given way to fish scales, but the color remained the same. Where her feet had been was single tailfin.
After looking her new form over, Ava gave a few tentative kicks then took off swimming. After a moment, you took off after her but it was soon apparent you had not chance of catching her if she didn't want you to. However, she soon circled back and swam up to you, now standing in almost neck deep water.
"How can I ever thank you?" she said with a mischievous grin on her face.