The Deep by Tim Winton
Alice's family lives by the sea. Every day they run down to the jetty and jump into the smooth, dark water. They look like a bunch of dolphins leaping and laughing. But Alice misses out. She's afraid of the deep. She's afraid of what might be down there where the water turns from green to blue and you can't see the bottom. Then, one day some new friends come into the bay and Alice forgets to be scared.