So then:
1. There were no sightings
2. Bloodhounds picked up no scents away from the beach
3. The current is ferocious, and one could get swept away at night, especially if disoriented.
This seems to point to her having to be in the water, just not found.
Perhaps she never drank and drove with her child in the car. Perhaps anti-anxiety meds, maybe taken due to the stress of the pending visit, disoriented her. The wine may have been drunk in a stopped van, parked on the beach to avoid being seen,(she may have looked around for the most out of the way road that would lead to the water; may have specifically chosen a private beach so there would be no patrol, less visible) with the plan of sleeping there overnight (then surprised by the tide, which she had not taken into account). She MAY have tried to do the best for her child under conditions of mental instability and ubearable stress. This may be simply a tragedy. The knock at one am, on the neighbors garage? I know many people do not believe in such, but there have been reports at the time of a stressful death, of strange bangings in the area, the disturbances of the atmosphere. In this sense, no one knocked, but the banging was simply the sounds coming from the charged atmosphere as mother and son met their death. It sounds "supernatural", but things of this nature have been reported, when the death is sudden, stressful, and tragic.