I am looking for tha name of a 1980's cartoon movie.There is a boy in it I believe he lives in a castle.The movie starts like a regular film then as sonn as the boy jumps into a pond on his property the film becomes a cartoon under water.Please help

The Water Babies (http://www.imdb.com/title/tt0078477/)

I have been searching for this same movie, I believe it is the pagemaster. Hopefully this is the movie you were looking for! =)