Low Tide — William Mayne

There had never been such a low tide at Jade Bay. It left fish on dry land and a wreck high on a rock, which must be the treasure ship the divers were looking for...

But three friend's own investigations carry them far into the unexplored mountains of the Knuckle, where the legendary wild man, the Koroua lives.

Lost and frightened, the children slowly learn that if strange things can happen that have never happened before, other things may not be what they seem either: not Korouas, nor treasure, nor even Jade Bay itself.

'A brilliant writer' Sunday Telegraph