Jeopardy

Doug and Helen Stilwin (Barry Sullivan and Barbara Stanwyck) along with their son Bobby (Lee Aaker) embark on a vacation into desolate Baja California in Mexico to a remote, deserted fishing spot along the coast that her father used to frequent with his old military buddies. At a roadside barricade heading into the Baja outback, they are inspected by police but are not told about an American killer who has escaped from prison and is at large in the area.
Upon arrival at the secret fishing spot, young Bobby goes exploring out onto a precarious, rotting jetty high above the water. His foot gets stuck in a crack between boards, and when Doug gets him out, the jetty collapses and a wooden piling falls on Doug's leg and traps him on the beach just as the tide is coming in. Helen nearly succeeds in lifting the piling with their car jack, until the jack breaks. They quickly determine that if Doug is not freed within a few hours, he will drown in the rising surf.
Helen leaves with the family car in an attempt to find help. She comes across a family of Mexican ranchers, but the language barrier prevents them from realizing that she is looking for rope until she is far gone. Helen then comes across a small house and finds rope, but seemingly out of nowhere, a man (Ralph Meeker) appears. Helen asks for help and the man gets in the car, but around the corner of the house, it is revealed that the man just killed a worker, and is the fugitive, Lawson, the police are looking for. He takes her hostage, and threatens to kill her if she reveals to the police that he is the fugitive.
Meanwhile, the rising tide back at the jetty causes Doug to have a heart-to-heart with Bobby, telling him he is proud of his son and hopes he will face the challenges ahead with strength. A boat passes close to their location, and shouts are exchanged, but thinking the family is just tourists on a beach outing, the boat crew does not stop.
Lawson runs through a police roadblock, and eventually blows out a tire. As he attempts to change the tire, Helen tries to hit Lawson and flee, but he catches her. Another police car catches up with them, but Lawson's driving forces the police off the road and the police car flips and kills the officers. Lawson hides out in an abandoned house to wait for the police to pass. While there, in an act of desperation and steely resolve, Helen offers to consummate with Lawson and go with him if that means saving her husband. Lawson, having just escaped from prison, takes her up on the offer, and the two show up to help Doug just as the tide is rising over Doug. Lawson latches the fallen piling to the family car bumper and tries to pull the piling loose, but this tactic is unsuccessful. Ultimately, Lawson uses a plank to wedge the piling off Doug, saving his life.
Helen offers to hold up her end of the bargain and leave with Lawson â thus giving him a pretext to escape as a couple â but he sees the goodness in her and escapes in the car alone. However, he turns back when he hears police sirens, and Lawson and Helen share an understanding handshake as he escapes on foot down the rocky coastline.

Question: What causes Doug to have a heart to heart with Bobby ?
Answer: The rising tide at the jetty.