Curt's mind was racing. He was too shocked to form a coherent thought. What was happening to them — turned into mermen? — was utterly impossible. This just couldn't be happening!
That train of thought was quickly derailed by a sudden surge of sensation that ran through Curt's newly dolphin-shaped lower body. More than simple pain or pleasure, it was like a tingle and a ripple and a few thousand needle-pricks all at once, running from his waist down to the tips of his new flukes. The feeling momentarily overwhelmed Curt, and he let out a low, gasping moan as more changes took effect.
Curt's tail retained its overall cetacean structure, flukes that moved up and down like those of a dolphin or a whale or a movie mermaid. But the surface of the tail, the spandex that had somehow become skin, was now mutating further, as fish-scales began to erupt through the skin and cover the tail. The tail's shade of spandex green became a silvery green that glistened in the light reflecting off the surface of the pool-water. At the same time, the flukes at the end of Curt's tail flattened further and grew considerably, morphing into the rayed fin of a great fish, though the fleshy membranes between the rays retained some of the heft and thickness of flukes; when the changes finally stopped, the overall shape of the fin suggested an impossible combination of whale and tropical fish. No other fins or members grew from the tail — no dorsal fin, no anal or pectoral fins — and at least so far, nothing about Curt's human upper half seemed to have changed.
The same could not be said for Bill, who had undergone all of the same continued changes as Curt. Bill now sported shimmering blue scales and a great rayed fish-fin that flopped involuntarily on the tiled floor by the poolside. But these rather superficial mutations weren't all that was in store for the androgynous twink turned merfolk. Their features softened and feminized further than they already had been; they acquired soft curves without losing any of that swimmer's muscle definition, soon sporting wide hips and a narrow waist in addition to their already toned abs; and their pecks blossomed into a pair of breasts, no larger than B-cup, still suited to hydrodynamically knifing through water at speed.
Curt stared, shocked, at what his friend had become. There was still a hint of androgyny there, but there was no denying the plain fact. "B-Bill," he stammered, "you're a girl."
As the mutations finished, and the weird sensations finally wore off, Bill was now able to look down at her body. Strangely calm, she cupped her new tits in both hands, and then felt down her tummy, down to the fish-scales on her tail. She grabbed her ass — the cheeks were scaly, smooth and even a little by slimy to the touch, and there was still an ass-crack there, though it was filled in now and didn't have a hole anymore — and then let her hands come to rest on the tiled floor, her arms locked at the elbows so that she could keep herself propped up into a sitting position. There was no doubt about it, and Bill voiced her agreement with Curt: "I'm a mermaid." Even her voice was undeniably more feminine now, and what was more, something must have changed in her brain too, because she was thinking of herself only with she/her pronouns.
"What — what do we do?" asked Curt, his voice sounding small and scared.
"I don't know," said Bill. Which didn't sound right to her — 'Bill' — no, she needed a new name. She was still the same person, just turned into a woman now, so something similar? Belle? Billie? Turn 'William' into 'Willamina' and shorten it to 'Mina'? She decided: "In the meanwhile, call me 'Billie'."
Curt blinked. "Uh, okay." But didn't they have bigger issues to deal with right now? Sure, he was curious about his altered body and how all of this had happened; but that could wait. Right now, he was concerned with two things: figuring out how to get somewhere safe where the two newly-transformed merpeople wouldn't be mobbed by curious gawkers, and how to undo this impossible body-horror.
And then there was still the concerning fact that those waves of sensation running through both of them, the aftershocks of the transformation, were still leaving Curt feeling . . . was 'horny' even the right word? How could anything like that even work with a fishtail? And the fact that Billie — Curt's friend, his swimming buddy, his bro! — was right there, was probably feeling similar feelings, and was a damned mermaid now . . . no, no, Curt wouldn't think those thoughts. Frustrated, he let his face fall into his hands; he rubbed his eyes; he took a deep breath to center himself. Gotta focus here. Gotta figure out what to do . . .