Katya Balen 
The Thames and Tide Club: The Secret City [EPUB ebook] 

An exciting new adventure series for young readers from Carnegie Medal winner Katya Balen, author of October, October and The Light in Everything

Clem and her friends Ash and Zara are members of the Thames and Tide Club! Every weekend they go mudlarking by the river, searching for treasures that have washed up on the shore. Clem has found old things, new things and a whole heap of rubbish. But one day, she finds something really special. Something magical that belongs in the river and must be returned to its rightful owner … or else.
Before they know it, the Thames and Tide Club are on the weirdest, wildest, underwater-iest adventure they could possibly have imagined on a mission to save Underwater London!