Charlie would've kept going at the rate he was going.
Would've merrily kept on swimming until he was nothing but a fish, swimming in and amongst the rest of his kind. Until he happened to shift his swimming form, and wound up seeing his arm. His arm, covered as it was in shining silvery scales, and his hand, thick webbing between his fingers, noting the curvature of his nails seeming distinctly different. Charlie's mind processed what he saw - as a human young man, seeing all of this. He almost choked as he screamed and surfaced from the water.
His lungs burned - and he realized in the state he was in, somehow, there were two different mechanisms for breathing now. Five sharp openings along his neck - gills! - and increasingly vestigial lungs. Needless to say as Charlie stood there, feet slightly further along than his hands, he almost slipped and fell due to the structural differences in his feet. But he also stopped his "one more round" through the salmon route as well.
He didn't hear the amused chuckle from the Fae watching him.
Paul meanwhile was having a freakout all his own. When he saw what emerged from the stream, he assumed he had been dropped somehow into some old horror movie. Of the monstrous Creature from the Black Lagoon rising from the murky depths and doing something - something horrible - to the people who were unfortunate enough to stumble upon its lair. The last of its vile kindred that dwelled beneath the ancient deep...
Whether or not he was badly interpreting the movie, one had to understand - Paul thought he was looking at a monster. The heavyset young man ruminated, I'm bi, young, and I left the safety of the group to hang out with my thrillseeking friend - I AM SO DEAD!! But his haphazard attempts to get back up and flee were met with something he didn't expect. More to the point, a voice he did not expect.
"D-Dude! I-I don't...wait! Stop! It's me! Charlie!"
That made Paul stop and turn around slowly, hoping this wasn't some kind of skinwalker situation. He bit his lip and halted his scream as the form of his friend slowly waded through the water toward the shore. It wasn't charging like a beast. Charlie was simply confused and scared himself - he had no idea what did this to him or what was going on. His monstrous appearance merely the coincidence of halting the transformation midway through.
"Dude...dude...Charlie? I-Is that you?" Paul approached cautiously, picking up a small branch from the ground just in case the creature was not Charlie.
"Y-Yeah...yeah." Charlie answered, looking warily at the stick. He knew well enough his friend was kind of hefty, and could throw a mean punch. So a swing from that stick would hurt, "Dude, I'm...I'm like, freaking out so hard here. I dunno...dunno what's going on." Now he had a chance to stop and take things in, he realized how insane this all was - and how different EVERYTHING would be from here on.
"C-can you prove it?" Paul looked cautious even still.
Charlie bit his lip slightly - realizing his teeth were halfway toward the configuration of a fish's - and said, "Cynthia Streng and I made out under the bleachers after my swim meet, and you stumbled on us just when I got my dick out."
"Oh my fucking god it really is you, Charlie!" Paul started to go forward, but stopped, and asked, "...You're not gonna like go all Black Lagoon on me, are you? Like, in all the movies when someone turns into a monster---"
"They also turn into a fucking douchebag." Charlie completed the sentence for him, having discussed that with Paul before. "I'm not about to go all instinctual and eat you or anything. I have...really, no idea no fucking idea how this happened. And I uh, I don't feel any different. Outside from breathing air feeling like such a fucking chore."
"M-Maybe you swam next to an irradiated fish? Or---Or there was radioactive waste in the water that uhh---"
"I'd be dead if either of those were the case, not well..." Charlie shrugged and gestured at his body, "Like this."
"We gotta tell the others---"
"They'd freak out dude! Like you did, but worse! Just imagine!" Charlie grimaced and shook his head, cupping his chin with his hand, "I-I don't...don't think I even CAN go back now. Just imagine how different my needs are gonna be. It's...really hard to breathe air like I used to, I think I need like a water tank or something."
Paul halted and realized what Charlie was saying was true. He had no idea what to do going forward, but for now he decided to just stand by his friend. He was...different for sure. But Charlie was very evidently still Charlie. Just...really shook up. "So...what? Are you just gonna stay out here?"
"I dunno man, I just...dunno. We gotta figure something out though," Charlie started, "I don't want to lose out on my life for---for just going for a swim!"
Paul looked around and said, "You feel hungry, man? I could uh, go get you something..."
Charlie realized as he felt his stomach angrily gurgle that he had worked up a lot of an appetite - and he had utterly no idea what he could eat. "You uhh, go try and find some food from the camping site. I'll see if I need to scrounge anything up." With that, Paul left...
But before he did, he shouted, "Dude, just...don't go swimming with the fishes any more, if that's what did it. I don't want to lose my best friend." With that Paul ran off to go try and find some food for Charlie. The young swimmer watched him go and realized he probably had a more faithful friend than he'd given him credit for being. And now he had some time alone and not shouting, Charlie could do some experimenting.
He got down on the ground and assumed push-up position - a little more difficult due to the deformation of his feet toward fin-like shapes, but he managed by adjusting his knee downward. He found after a moment he could do as many push ups as he used to be able to do, meaning his strength hadn't been impacted as of yet. After getting back up, Charlie thought about the girl he liked.
Cynthia and...the fact if they were to have a relationship going forward, she would have to be willing to engage in the plot of that one movie where the chick fucked the sea monster. Still, Charlie was a young man - and still had his healthy libido. So he started contemplating fucking her, feeling her hands brush the fins down his back, shivering with pleasure as she kissed him. Slipping out his dick and---oh. Charlie felt his arousal coming on despite the hunger.
But it was not as strong or as obviously large as usual.
He pulled down his speedo to see that his manhood was just about half its usual size, and oddly formed. Definitely not like what he was used to, and definitely not as large. "Aww that just isn't even fair! I lose my size!?" He ranted aloud, and sighed. No point in complaining now. He needed to eat, and then he could deal with his issues. He hoped it was still...well, responsive. He started to look around. There was no allure in any nearby edible berries, and he didn't much care for catching food.
He plunged into the stream again to try and find some food - and found that there were small creatures swimming along that drew his attention. He made himself open his mouth and take large swallowing gulps of these small creatures. It took more than a few gulps, but he was before long feeling satiated and able to leave the stream.
Getting up and walking out of the water again, he grimaced as he realized what he'd eaten. Water dwelling bugs and worms, in all likelihood. But Charlie was willing to do what needed to be done to live.
----------
Una and Puck watched the ensuing feeding, and Una noted how the boy seemed so reluctant to make use of his aquatic abilities. Possibly out of fear of continuing on his transformation. Much as it might've been disappointing in the moment he discovered her trick, she now felt amusement and interest renewed. She ceased her spell on him that would've continued his change into a salmon, interested now in how he might respond to his current state.
And she did not rule out the possibility of affecting his instincts.
"You adjust well to twists and turns." Puck commented.
"You cannot be successful among the Fae without adapting to the situation. Charlie is in a very precarious spot. Perhaps Paul will be receptive to a change of his own..." Una responded, already thinking of what she may be able to lure the other young man into.
Both Fae laughed, and began contemplating the course...