It was slightly awkward, Kyle couldn't move his lips away from Crystal's mouth without drowning, and he had to make sure not to breath through his nose, but on the other hand Crystal was a good kisser and the way her athletic body was rubbing against his made him start to get stiff underneath her. Lifting up her hips, Crystal reached down and guided Kyle's length into her. He could feel her moan out as he slid into her willing body and she slowly started to rock her hips up and down.
As she did he started to notice a certain roughness to her thighs as she rubbed against him. He realized that it was probably those scales she'd mentioned growing earlier. Though hadn't she said that they usually took about an hour to show up?
The pace of her pelvis increased and Kyle banished those thoughts from his mind and instead focused on matching her. He let his hands explore her body, making trips to that erogenous zone he'd found along her dorsal fin which seemed to excite her even more. After everything with Rebecca and Samantha he didn't know how much longer he could keep this up when suddenly Crystal started to buck against him and the moaning coming from her mouth intensified. Quickly she started dragging him up to the surface with her. When she finally reached the surface she broke the kiss with him and leaned back to start to moan. "Oh fu- ohh! Something's happening."
Kyle could barely tread water in his current state but tried to swim over to her. "What is it? What's wrong?"
"My legs! It feels... fuck it feels incredible!" As she said this Kyle looked down at her legs and saw that they looked to be stuck together. As he watched they seemed to stretch out while fusing into one long limb. Finally he saw the scales she'd grown on her thighs start to spread. A lovely bronze color they soon covered all of her tail (which it clearly was now) as her feet spread out and flattened into a pair of fins.
Other changes took place too. Crystal's hips seemed to become wider, and her breasts swelled forwards to the same region as the frog-girls inside. With one last scream of pleasure she fell backwards in the water and just floated there for a moment. "Oh wow," she said to Kyle. "Oh wow I'm a... I'm a full mermaid."
"Did you-?"
"No. No as far as I knew this was impossible. It's not the first time I've had sex. Maybe it was having sex in the water that did it? Or maybe it's something left over from you being froggy?" She shook her head, "Who cares? This is amazing!" She playfully splashed at Kyle and dove back under the water.
Just then...