I believe the movie you are thinking of is "The Water Babies".
"...MGM's obscure 1978 live-action/animated family flick The Water
Babies. Though largely forgotten, some fondly recall this magical
story of an apprentice chimney sweep who goes on the run after being
wrongly accused of thievery. He falls into a pool of water and is
sucked into an enchanted animated underwater world populated by
fantastic characters and creatures. Tom must eventually decide whether
to return to the human world or remain in the realm of the Water
"Plot: England, 1850. The street urchin Tommy is taken by his bullying
employer Grimes to clean the chimneys at Harthove Hall in Yorkshire.
Grimes uses the opportunity to rob the house's silver, but when he is
surprised by the owners of the house he shoves the silver into Tommy's
arms and cries "thief". Tommy runs scared and falls into the river and
is sucked under. But there Tommy discovers that he can breathe
underwater as well as talk to the marine life. Seeking to find a means
of returning to the surface, he heads upstream to find the water
babies, other children who have likewise been swept under. But there
an evil shark captures the water babies and Tommy must choose whether
to return to the surface or whether to relinquish the chance to do so
and become the rescuer of the other babies."
THE WATER BABIES
More on The Water Babies at the Internet Movie Database:
This movie is available at Amazon:
site:.imdb.com boy falls animated underwater
I hope this is helpful. If you have any questions regarding my answer,
or believe this is not the movie you are looking for, please don?t
hesitate to ask for further assistance before rating.