The sand on the shovel was supposed to come from a place where a bonfire had been. When we see the victim buried, it is under a pier on the beach. There is no pier in the opening scene where we see the accident happen. It would also not make sense to build a bonfire under a pier.
When the crime lab runs a DNA comparison to find out who used the crane remote, they get a 96% match with the victim. They determine it is a familial match, and this leads them to the victim's son. Since we only get half of our DNA from each parent, there should have only been a 50% match at best.