One boy (12) is briefly seen frontal nude after other boys have pulled off his clothes and they throw a stone at his face, so his hands go to his face and you see him completely nude. Before that, you see a glimpse of his penis while the boy is wearing his shorts. A number of characters (aged 10-15) are seen naked from the rear.
After running down the stairs, he is caught by a woman who pulls off his towel and spanks him on his bare butt. Brief frontal nudity is seen when he falls down.
When the boat runs out of fuel, Hannes takes of his pants and underpants so they don't get wet when he brings the boat back to shore a girl is watching him.