Margaret Sutton 
Puzzle in the Pond [EPUB ebook] 

When redheaded Judy Bolton and her friend, Holly Potter, set out after the green car, they never dream that the chase is going to lead them into a far greater mystery than just the theft of Holly’s typewriter. What Judy sees under the surface of the pond by the beaver dam is enough to mystify-and terrify-anyone, and the young detective can’t be blamed for not immediately realising the significance of the "apparition." (Goodreads)