The girl and the mermaid by Hollie Hughes

Alina and her granny live in a lighthouse by the sea: a home filled with the warmth of Granny's magical stories. But Alina is worried. Granny's memories and stories are starting to fade away, and she doesn't know how to bring them back. Then one day, she meets a mermaid on the rocks near the lighthouse - and she's swept into a magical deep-sea adventure that will change everything.