As you look back you see that your bikini bottom has fallen off and that your legs are growing together. You see scales growing over your legs, and your feet growing together into a flipper.
You try to scream, but you are unable to make a sound. You can't breath. You run to the water while you still can, just making it in before you loose what little mobility you have left.
You float down under the water, and for the first time you look at what was once your cute vagina. You see nothing but flat skin and a now enlarged clitoris.
Before you can move, you see that your clit keeps expanding, and the flat patch under it is bulging out. Before you know it, you have the dick and balls of a teenage boy.
As your genitals change, so does your body. Your pubes grow out, your muscles grow, and your breasts shrink (though your nipples remain the same hyper-sensitive nubs that you're used to). Before you can finish looking around, you are a well muscled, well endowed teenage boy.
You look down in disbelief at your cock as a single mermaid, swollen belly and all, glides down to see you.
"What the hell just happened?" you ask.
"We saw you watching, and we had no choice. You see, we need you. We may look pregnant to you, but in reality we are simple fertile. We have eggs, but we need your sperm." responds the mermaid.
"I don't want to be a boy!" you exclaim.
"We understand that this won't be easy for your, but for now, we will wait for you to become accustomed to your new body," responds the mermaid, "when you have completed your task, we will return you to your human form."
She begins to swim away, but spins back. "Oh, I almost forgot...We didn't complete the change...Just one thing left to do."
You feel a rush of arousal and stiffness of your new cock as the hormonal insanity of male puberty sets in. As fast as you can, you rush back into a dark corner, where you try to get this under control
You reach down and grab your new cock, and it swells to its biggest size...almost 12 inches long. It's so sensitive. Every little touch makes you groan. It's so biiiiiiiggggg........ You can't take it any longer. You rush back out to where the mermaid it waiting, and without a word, you grab her and shove yourself into her. Your long hair, the only remains of your female self, floats around you as you shove deeper and deeper into her. You feel your balls contracting as your flippers slap each other. You grab onto her back as you shoot a massive load into her pussy, draining yourself. As you float, your body returns to its feminine form, but, for now, you can still breath.
You watch as the mermaid in front of you grows something like a cock, but massive, almost a foot across and at least two feet long. You feel your female hormones rush in as you grow horny again. She moves toward you and her pseudo dick grabs on to your pussy as you feel something draining into you. You come harder than you ever have before, muscle contractions pulling the now fertilized egg into you. As your orgasm dies down, you collapse onto the sea floor.
Before you know it, you are lying on the floor of your room, with the mermaid, now a complete human looking female, standing next to you. You pull yourself up, looking down at your slightly bulging stomach.
"For now, I will wait with you, but it is too dangerous for your mind to be in that body right now. If you remain there, the egg may be rejected. We must give your body to someone else," the mermaid said.
"Why?"
"Because this baby is completely of your own body, just with a few genes altered to be like ours. If you remain, your mind will see it as a mutation and reject it. I have seen into your mind, and I have found a perfect candidate to switch you with. I will stay with him as his girlfriend, while he comes to term and delivers the child."
"Wait," you say, as you feel your body starting to fade away,"what do you mean, he?"
In a flash of light, you leave your body.
"Wow" says previously your body "what the hell was that?"
You look down to see your sister's body below you. You see the large boobs and, to your amazement, a pussy down below. Before you can do anything, you feel a growing arousal inside you.
"I've gotta jack off...er...masturbate right now," you say as you run your hands over your sensitive boobs. You lay there, a boy in a girls body. Your body has become a girls. You have switched bodies with your own sister, and she's not here to tell you what to do. You feel what you somehow know to be an egg growing inside you. But you don't think about that, because, for once in your life, you can feel what a girls body feels like from the inside.