All that I remember is that it's from the mid to late 80's, I think. A boy gets an idea from throwing a toy into his fish tank than he gets trapped in what everyone thinks is a monster, really it is a backhoe in the lake or swamp or pond or something. Know what it is?

Frog Dreaming (1986) (http://www.imdb.com/title/tt0091810/)

The Quest