The Water Babies (1978)

Grimes, an amoral chimney sweep, occasionally likes to steal valuables from his clients. One day, on the verge of being caught, he frames his young apprentice, Tom, for the crime. Tom runs away and jumps into a river where, instead of drowning, he finds himself transformed into a mystical aquatic creature. Swimming and breathing effortlessly, he discovers a colorful underwater world replete with creatures both cruel and kind.