Children's magical underwater adventure - solved [s]

[Esther]

In this children's story a little girl is walking by a river and somehow she understands that the river is in danger. Someone creates a magic mask (using leaves I think) and after she puts in on she can breathe and live underwater.
She lives in the river with a magical collection of taking fish and water animals until she removes the curse.

[Sue]

Maybe "The Stream That Stood Still" by Beverley Nichols.
The girl was called Jill, and her brother, Jack, had been turned into a fish by a witch.
Her Grandmother made her a breathing mask of waterlily petals.

[Esther]

That is exactly it. Thanks so much.