It all began on Percy's island. Percy was standing on the beach near his house.
Prior to his transformation, Percy was a man with the aforementioned features, with a stout body structure and round belly, wearing a blue jacket over a green turtleneck sweater, grey sweatpants, and black boots.
Percy stood on the beach, looking out longingly into the calm cool ocean water ahead.
He breathed the sea air in through his nostrils.
"Weather's lookin' fine for a nice dip." he said.
Without a second thought, Percy went to his changing booth he'd build near his house, so he could change into swim gear and back easily.
He changed outta his clothes, then put on his stripy swim trunks.
He exited the booth, made sure his house was locked up, and booth too, and then he went back to the beach, walked to the shore, and dipped a toe in the water.
It felt cold, but it didn't matter. he waded into the water, and then submerged himself.
It was then Percy noticed something surprising - he was breathing normally under the water.
"Huh." he said to himself. "I feel kinda funny. It's not everyday humans can breathe underwater."
He scratched his head, and then realized his hands were every bit different than normal - the fingers were much thinner, and had webbing in between them!
And, upon seeing his feet, the same was true for his toes.
"Holy shit." Percy said. "I got meself webbed toes and fingers."
Then, his legs began to shrink, and curl into his body, and within seconds, a huge pale blue tail appeared in place of his legs, making his trunks vanish outta existence.
"Wow." Percy exclaimed. "I'm a merman!"
Percy the merman swam in the sea for a whole hour.
but, near the end of the hour, he changed again - his arms shrunk and his hands changed into fins, and his body became less human and more fish.
Until, he was turned into the human-faced fish Johnny and his brother saw.
His human face remained the same.