Everything Under by Daisy Johnson

It's been sixteen years since Gretel last saw her mother, half a lifetime to forget her childhood on the canals. But a phone call will soon reunite them, and bring those wild years flooding back: the secret language that Gretel and her mother invented; the strange boy, Marcus, living on the boat that final winter; the creature said to be underwater, swimming ever closer.

In the end there will be nothing for Gretel to do but to wade deeper into their past, where family secrets and aged prophesies will all come tragically alive again.
 
'Weird and wild and wonderfully unsettling... Dive in for just a moment and you'll emerge gasping and haunted' Celeste Ng, bestselling author of Little Fires Everywhere