You feel the magic of the cold water surround you, pushing your legs together. Your feet merge together, transforming into fins. Scales appear at the base of your fins, climbing up to just above your knees. You then feel the flesh of your ankles merge together, then your shins, and, finally, your knees. A couple of scales appear on your thighs and arms, but not a lot. You feel the magic of the water retreat, and you climb onto the rocky ledge.
You look at your new tail, which is a shade of blue. It's not quite what you expected. For one, you kept your human joints, while you'd been expecting to have more dolphin-like joints, like Joan. For another, your tail ends about six inches above your knees, not at your waist, leaving your cock and balls free.
You slide back into the water, and swim to the mouth of the cave, where Joan is waiting. You kiss her.
"You can change back anytime you want," she tells you. "They told me that when I first changed."
"Okay," you tell her. You kiss her again, then swim off to explore. You notice that many of the merpeople have tails like you do. After exploring for a while, you begin to feel bored...