The island that wasn't there - Ruth Snowdon

Rosie often imagines that the dense cloud, which she sometimes glimpses in the middle of the sea, is an island. One day, after a violent storm, Rosie finds a mermaid, Anemome, tangled in the seaweed. The mermaid is too scared to swim back to her home in case she's caught by the Finfolk - men who capture mermaids (and humans). But when Rosie tries to get help for Anemone, no one believes her - except for an old man called Yan Eye. Yan Eye agrees to take Anemone home in his boat, and Rosie insists on coming too. But in the middle of the ocean, their boat is surrounded by sinister-looking, ghost-like people - Finfolk! Rosie and the others are captured and taken to an island in the middle of the sea - an island that wasn't there...