Not quite a mermaid - Linda Chapman

Electra and her school friends are off on an overnight trip to Craggy island. When they arrive, Electra sees some of the older mermaids looking at her and giggling and she is upset to discover they're laughing at the fact that she has legs instead of a tail! The girls are even meaner later on when they tell Electra and her friends that the island caves, they plan to have a midnight feast in, are haunted.

The next day the older mermaids swim off to the rocks where a special moss grows that will make their hair shine. But later on, Electra sees that the girls have got stranded on the rocks and their tails are burning in the sun! Even though their teacher told them not to approach the rocks, Electra decides to go and help the girls as she is the only one who can because of her legs!

The older girls are so grateful that they show Electra and her friends an underwater grotto, lit up by mermaid fire with tiny pink jellyfish and seahorses where they all have the best midnight feast ever!