"The seal was close now, swimming parallel to the shore.
“Enter,” it said, “Enter”.
She watched in amazement as, starting at her toes her feet, then legs, began to turn silver. She could feel under her clothes the rest of her body changing too.
She pulled off her trousers and pants, then her top. Her whole body was silver now right up to her necklace. Beautiful iridescent silver scales. She looked down. She could not move her legs apart, her feet were becoming fused together into into ..a tail.
She felt an urge, a terrible urge to slip into the sea. She turned and without fear slid into the water. It was not cold to the touch but warm. She began to swim out to the seal.
As she neared, it spoke.
“On the land you are a Finn girl, here in the sea you are a mermaid, you can swim wherever you wish but beware of the currents. I will teach you."