At 14:55 at the beginning of the swimming scene, Joe Harper (Dale Walters) is on a platform to the right of a tree. Huckleberry Finn (Ian Tracey) is on the platform to the left of the tree. Between 14:55 and 15:00, Joe moves behind the tree and then to the left of it, facing the camera with his arms crossed, as Huck moves to the left on the platform towards the edge and stands up to dive. Tom is below the platform holding a rope swing the entire time. In the next shot at 15:00, Huck is about to dive and Tom is still below the platform holding the rope swing, however, Joe has disappeared. Between 15:00 and 15:02, Huck dives off the platform and into the water. When the next shot begins at 15:03, Joe has reappeared on the platform, again facing the camera with his arms crossed in almost the same position as he was at 15:00 in the first shot.
At the point where Tom, Huck and Joe are watching the paddle-steamer and see (amongst others) Joe's mother and Aunt Polly on board, Tom says that both women are crying. There is no way that he could possibly have seen that level of detail from that distance.