The tide knot by helen dunmore

This is the second volume of the book Ingo. Sapphire and Connor are learning to cope since the disappearance of their father, Matthew Trewhella, into the sea. Mum has moved the kids from their cottage on the cove to the bigger town of St. Pirans. Mum and Connor are getting on well with the new environment, but Sapphire is unable to adapt to her new life and having trouble to forget about disappearance of her father, she is now spending more time beneath the waves with her Mer friend Faro in the world of Ingo.
On her adventures with Faro, she meets his wise teacher Saldowr and is shown the Tide Knot - a large stone which controls all the tides in Ingo.
One night, however, the Tide Knot is loosened and a major flood rushes into St Pirans, trapping the Trewhella family indoors! Will Sapphy and her family escape? Will faro and the Mer come to help or will the Mer take revenge against the humans for what they have done to the watery world?