Between the Two (1966)

Plamen's parents are separated. His father is an old river wolf, his mother a nurse in the big city. The boy's longing for his father keeps him going. His studies don't go well. One day Plamen sneaks onto his father's barge and secretly sets sail with him. On this long and very interesting voyage, Plamen befriends his father, and when they have to return to Varna, the boy is happy: he hopes that all three of them will be together again.