Emily Windsnap and the Castle in the Mist by Liz Kessler

When Emily Windsnap discovers an old diamond ring buried in the sand, she has no idea that it belongs to Neptune, King of the Sea - and he wants it back.
 
But the ring, once on, won't come off. With a furious Neptune on her tail, Emily finds herself under a curse. In just a few days, she'll cease to be half-human and half-mermaid and must say goodbye to one side of her life forever.
 
She needs to find the matching ring that will break the curse, before it's too late...