The Water Babies (Collins Classics) - Charles Kingsley

In fact, the fairies had turned him into a water-baby. A water-baby? You never heard of a water-baby. Perhaps not. That is the very reason why this story was written.

Poor Tom is an orphan, employed by the underhand Grimes as a chimney sweep. While cleaning a chimney one day, Tom becomes lost and appears in Ellies bedroom. She mistakes him for a thief and he flees in fright, down to the edge of a stream where he falls asleep. In his dreamlike world, he becomes a water baby, swimming with the other water babies and discovering an aquatic playground full of strange and wonderful characters.