Real movie turn into cartoon

I don't remember much about this kids movie(I think it is), but I hope you Guys can help me anyway.

The Movie starts out with real persons and the main character is a poor boy. Then he either fall or is pushed into a lake or the sea...
While he struggle under water the movie turn into a cartoon. From then on he can breath under water and get himself into some kind of adventure where he talk to fishes(I think, since I mostly recall the start and the part where it goes from live action to cartoon). I do think it goes back to live action in the end of the movie. I'm not sure, but I think it's from the 80s.

Hope that's enough to find the title of this movie.

Thanks in advance.