"I wasn't always a mermaid," you explain. "I only became one today. Yesterday, I was a guy and went to the beach where I found a group of mermaids. Then when I woke up this morning I started turning into one...or at least a pool float that looked like one. Then you came along and changed me back into a real...mermaid."
"You're welcome! Although I don't know how I did it."
"Neither do I. Something about the way you were groping me, I guess?"
Katie blushed. "I was feeling a little horny, and mermaids turn me on."
The hot neighbor girl is turned on by mermaids? Maybe you're not in such a hurry to get your human form back after all. That's when you notice Katie staring at your bare breasts. You cup them with your hands.
"So you like what you see?"
Katie bit her lip and nodded. You swam closer.
"I want you to help me find a way to return to being human...but we can have some fun together first."