On Silver Tides - Sylvia Bishop

Kelda has always protected her little sister Isla from danger on the rivers, and from the suspicions of their community. For Isla cannot breathe underwater – and so her very existence is forbidden by silvermen’s Lore.

Now the rivers of England are growing sick: monstrous creatures are awakening and a fierce torrent of blame falls upon Kelda’s family. When betrayal comes, the sisters escape on a desperate journey upstream. Joining forces with a mysterious boy, Kelda discovers the darkest depths of her kind’s secret history. But to save both her sister and the very life of Britain’s waterways, Kelda will have to make a sacrifice – one that will change her life forever.