Charles Kingsley 
The Water Babies 

When Tom, an ill-treated little chimney-sweep, jumps into a clear, cool stream to clean himself something magical happens; he is turned into a tiny water baby by the fairies. He enters a strange, magical underwater world, and travels beyond the world»s end to the other end-of-nowhere, getting into all sorts of scrapes and encountering creatures beautiful and frightening along the way. He also learns many important lessons – it is a voyage of discovery that Tom will never forget.