I freaked as I dropped under the water. I was a terrible swimmer, especially against a current! I was gonna drown! But all of a sudden, something grabbed me, and I looked across to see it was a mermaid. I was sure I must be seeing things, but she touched my cheek, swimming upwards with her tail, and spoke to me in a voice as clear as if we were above the water.
"Don't worry, human," she said. "I saw your wish to be of the other kind to your own. I will bring you to land and make you the other kind!" Soon we seemed to reach the shore, and she kissed me softly before practically throwing me onto dry land.
I coughed and gasped for air, equally shocked and relieved to still be alive. Once I could breathe comfortably, I noticed a weird tingling sensation all through my body, and assumed it was something to do with my waterlogged clothes. I looked around to make sure no one would see me, and pulled them off- I carefully removed my shoes, socks and trousers, and laid them on a dry rock.
As I pulled off my shirt, I noticed something- my hair fell in front of my eyes, and I felt like I had to brush an absurd amount of it away. It was much longer all of a sudden, I realized. And looking down at my body, it looked totally different. My skin seemed softer, what little body hair I had had had vanished, my arms and legs and hands and feet all seemed more petite, and my hips seemed wider.
Practically as soon as I realized how much I'd changed, I started to change more. My nipples had gotten hard from the cold of the water, and now they were swelling up. Before long the tips were as wide as the whole nipple had been, and the areolae were much wider. I bit my lip as I felt flesh start to grow out around them; I was growing breasts! The mermaid really was turning me into 'the other kind from my own'- from a boy to a girl!
I peered past my new boobs and saw my penis, and shimmied down to the edge of the water to see the reflection of it changing. A twinge shot through me, and I moaned, my voice cracking as I ejaculated for the last time. My white seed squirted into the stream, and I watched my shaft slowly recede away, not as an erection softening but as the flesh contracting. My balls tugged up inside me and reshaped into ovaries, and my scrotum flattened into me until it sifted into folds that formed a vagina. The protrusion which had been my cock had now shrunk to a small sensitive nub and formed my clitoris.
I stared at my reflection for a moment in amazement; I was a girl now.