This One Is Partly Animated And Partly Not, All I Remember Is This Little Boy That Ends Up Going Down Into The Ocean Or Just A Lake Or Whatever And It Is This New Land And He Has So Much Fun, I Can't Remember I Think He Might Have Became A Fish But Maybe Not, But He Had This Magical Day In The Water And At The End Of The Day He Goes Back Home, I Remember Something About A Piano For Some Reason But I Don't Know, So Anything Please Help!! Thanks

Was it possibly "The Water Babies" from 1978? It was a partly animated / partly live action fantasy that centred around a 12-year-old boy who discovers an underwater world where young children are held prisoner by an evil shark and an eel. Here's a link:

The Water Babies

Cheers... Mel.

You know I think that is it!! Thanks so much!!