So your girlfriend's a were-mermaid?
As it happens, you're pretty jealous.
"I- This is- How did this happen? You have to tell me!"
Katie shrinks back, looking worried. "Bobby? You don't hate me, right?"
Crap, you scared her. Good job. You push towards her and wrap your arms around her. (You'd wrap your legs as well, but she'd notice your boner, which she might not have a place for. You don't know yet.) "Hate you? Katie, this is awesome; I can't believe I'd ever meet a real mermaid! ...Well, were-mermaid, I guess, but still! You've got to tell me what happened!"
Your girlfriend pulls back, hesitating. "When I was 12, I was out diving around here. I could swim okay, but I didn't know my limits for diving. I was underwater for too long without air; I think I hit my head on something. I blacked out, but somehow I came to and I saw that a mermaid was kissing me! Then I realized that I could breathe, and I looked down and saw that I was a mermaid too! She told me that she was a full mermaid who could change at will, and that if a mermaid kissed an underwater human, the human would become a were-mermaid. Apparently, a were-mermaid can control which form she has most of the time, but she's always stuck in mermaid form during every full moon."
Gears turn in your head, but you hold off on that. "So you've been like this since you were twelve?"
"Oh, yeah. I won't lie; it was tough a lot of the time, but it works out nowadays. Except for with you. I'm sorry I never told you."
"It's okay. I do wish you'd told me or shown me earlier, though."
Katie cocks her head to the side. "Why's that?"
Here goes. "I'm kind of jealous. I've always wondered what it would be like to be a mermaid."
Her eyes widen. "Bobby... I never knew..."
You shake your head. "It's okay. It's not as if you should've known, right?"
"But..."
"If I wanted to be a were-mermaid with you, would you change me?"
Katie's eyes narrow. "Bobby, this isn't something you should take lightly, okay? There's no going back on this - every full moon, you'll need to drop everything and find a large body of water to swim in. A bathtub won't work, and while you can just lie around in bed or whatever, you'll get bored and frustrated quickly. Trust me." She takes your hands in hers and locks eyes with you. "Bobby. Are you sure you want to do this?"
You smile and peck her on the lips. "I've wanted this for so long. Please."
Your girlfriend manages a weak smile of her own. "Come on, then. We have to be underwater." Water hits your face as she splashes down below. You quickly dive down after her.
You keep your eyes closed, since even the full moon isn't bright enough to illuminate the water. Soon, you feel movement through the water, then you're being embraced, arms around you and breasts against your chest. You hug back, assuring her.
Then her lips meet yours, and it's all you can do to return the kiss.
She pulls away, and you feel yourself transforming rapidly. Your body feels soft and distant for a moment, and then you're back. Except "back" is a lot different than what you're used to - you're feeling different above the waist as well as below.
Well, you can breathe now, it seems. Opening your eyes you find you can see quite well, although it's a lot different than what you're used to. You can probably see in infrared light now, you hazard. Eager to view yourself, you look down and see a pair of miniscule peaks of breasts trying to impede your view of your tail. They do a poor job of it, though. From what you can tell, it looks the same as Katie's, though it feels kind of odd to have. Not having leg muscles, walking won't quite work, but moving your ass seems to have an effect. You're also not feeling any familiar male equipment, but you do notice an odd muscle in your tail where your crotch had been. You've a good guess what it is.
That wasn't quite what you'd planned. Looking at Katie, she seems as surprised as you are. Her expression changes to one of mild terror.
"Bobby... I never meant- Don't hate me, please!" She turns away, on the verge of tears.
With some effort, you swim over to her and grab her from behind, spinning her around and squeezing her. You feel your breasts against hers - a jolt passes through you, but you ignore it for now. Still, this is another sign of how good this is.
"Katie, it's okay. I wanted to be a mermaid, right? I knew that if I might have to be a mer-maid-, it was a chance I had to take. Besides, even if I'm stuck as a woman, it could be pretty good." You kiss your girlfriend on the lips. "Don't cry, sweetie. I'm not going anywhere. I hope you don't either, but I'll understand if you do."
Katie looks up at you. "Go?"
The look on your face is bittersweet as your gaze meets hers. You're prepared for the worst. "I'm a girl now, right? I still like girls, but you probably don't. I'll still love you, but if you can't love me that way anymore, it's okay."
You feel your girlfriend's arms crush your abdomen. "Oh, Bobby, don't say that! I love you just as much as you love me, even if you're a girl!"
"Do you really mean that?"
Katie looks at you. "Of course. We even have this between us now - this secret just for us." She nuzzles your chest, and you revel in the sensation. One of your arms curls around her shoulder, the other between your pressed-together bodies, migrating down to your tails. You stop at pelvis-level as your fingers tug at the cleft in Katie's tail as well as the one in your own. You both jump, but you more so, still not used to these feelings. You're aroused and infatuated nonetheless, though. You stroke Katie's cleft, pleased with yourself as she squirms. Your eyes meet again, the desire apparent.
"So... How much longer is the full moon for?"