L. Frank Baum 
The Sea Fairies (Annotated) [EPUB ebook] 

• This edition includes the following editor’s introduction: The life of L. Frank Baum, the father of Oz, a fall from success to disaster

Originally published in 1911, 'The Sea Fairies’ is a children’s fantasy novel written by American author L. Frank Baum. Baum is best known for his children’s books, particularly “The Wonderful Wizard of Oz” and its sequels.
As an underwater fantasy, Baum’s 'The Sea Fairies’ can be classed with earlier books with similar themes, like Charles Kingsley’s 'The Water-Babies’, and successors too, like E. Nesbit’s 'Wet Magic’.
'The Sea Fairies’ chronicles the adventures of Mayre 'Trot’ Griffiths, a small girl living on the coast of California, and her close friend and companion Cap’n Bill, formerly her father’s employer and now a retired seaman with a wooden leg. As they walk along the beach one day, Trot wishes that she could see a mermaid. Nearby mermaids overhear her, and grant her wish, appearing to her and the Cap’n the next day. The mermaids explain that they are 'sea fairies, ’ and offer the girl a chance to experience their world. Trot is eager to go; Bill is not, but is too loyal to let Trot go by herself. Magically transformed, the two embark on an underwater adventure.
'The Sea Fairies’ was followed by a successful sequel published the following year, 'Sky Island.’ Both books were intended as parts of a projected long-running fantasy series to replace the Oz books.