"Hey Mark!" Body #6 says, "I have it downstairs!"
"Oh, cool," he says. He's a well-built guy, from years of swimming. As you take his bag, he walks downstairs to your room.
"Wow," he says, "I haven't been in here in a while. Not much has changed, I see."
"Well, the bed's bigger, but apart from that, nothing else," you say from on the bed. You're wearing a two-piece wetsuit, each body having a different color. "Anyways, come on up!"
After an embarrassing amount of time before you just pick him up, you show him the pendant.
"What's this?" he asks
"I found this in the ruins we found last time," you say, "And while I'm not fluent; this should let you swim infinitely."
"Really?"
"Yeah. I just found it this morning, though, so I haven't tested it yet."
"Let's do it then!" he says, putting it on himself. You head out to the lake, where Mark is being a big baby about swimming.
"Just come in!" you say, "Or I'll push you in."
"How are YOOOOUUUUU!" Splosh!
Like a wave, he starts changing. His dirty blond hair lengthens and starts to flow with the waves. He moans, his voice shifting to an arousing feminine one, and his face makes all 12 of your hearts stop. You quickly drag her out, but her slim shoulders show no signs of stopping. Two globes sprout out of her chest, her nipples fading into a patch of scales. Her stomach flattens, her waist slims, and her hips sprout out. Her legs fuse, skin turning into golden scales, and fins appear at the bottom. She looks at you with a mix of arousal and fear, and she moans out, "Whaat's happening to me? It hurts!"
"What hurts, how!" you say, half panicking. This wasn't supposed to happen!
"Here," she says, half moaning, "It burns..."
"Oh, you're just aroused. And a girl."
"I'm a what!" she says, opening her eyes. "How... Why..." She flips her tail up. "I'm a mermaid."
"Yeah, you are," you say deadpan.
"Put me in the water. NOW!" She's gone from aroused, to horrified, to overjoyed in less than a few seconds. You dump her into the lake, and decide to deal with it later.