I've posted this a few times!
When I was little I had a book.
Agirl walked to the sea at night and the tide ahd gone so far out that it had revealed a city that had been drowned.
She explores it and the tide starts to come back and shes running to get away from the water.
Its very eerie!
I can't remember if she meets anyone there... please help me remember this I really want to find it!