She resurfaces and perched upon a rock. Her breasts were now covered in golden scales, her hair was over twice as long, and below her waist, was a golden mermaid's tail. Even though she was your stepmother, there was some level of attractiveness you struggled not to acknowledge.
"You're a... m-m-mer-- maid!" You gasp.
"Yes, I am!" She said.
"Does... Dad know about this!?"
"All too well at this point. Your father is now enjoying the same life I do; the same life I offer you now."
"You want me to become..."
She nodded. "Just like us. Think about it; everlasting youth, a magical voice, never to know sickness again!"
"Why would my Dad agree to this without asking me!?"
"A valid question. Your dad knew about your interest in merfolk. We agreed this would be an easy decision for you to make."