Agree it should be that simple, but reports especially the last one on 8th July 1988 says they don't know how it got there but the lights and ignition were turned on when it entered the water. The back seat was found two days before the car entered the water (unless you are convinced the car hid for two days). It would have been easier to dispose of car anywhere else, but it did end up in ocean at Cott and maybe how it got there is not so important.
Sand drifts covered the access road to groyne, and there was an access driving past the boatshed and making a sharp turn south, Reports said there was a one metre drop from the promenade to the sand.No police reports mentioned John St and besides the steps from what I can remember this access was blocked with a locked swing gate.
Fiat weighed 1.150 ton dry and would be heavier once it reached water, and had an estimated bouancy factor of 10 (similar to a 100kg person having 700kg on top of them). It could not have washed out IMO, any waves and tide would be pushing it into the shore. Photos of a 4WD (car V ocean) show a car fitted with snorkel exhaust and driven fast into ocean, not to be able to make it more than 5 m.
As I've said the water wasn't deep, my estimate would be 2 to 3 metres deep between pylon and groyne, almost always clearly see bottom and overhead images show the ocean bottom very clearly. I think it was seen within hours of it entering water. I think they wanted it found, so the lights were on. There is a multitude of people using the beach. Clubbies don't sit in the club, they are surf club users like myself who would swim, ski or board paddle every day before work, bobbies swam in the middle of beach, many joggers used the beach and the paid life guard or beach inspector can be confirmed by Council and would be on the beach by 6 or 7 am every week day.
The mybeach link shows the current paid lifeguard hours and also mentions The more protected southern corner usually has lower waves and weak currents and is popular with families and children. This is also very good for showing live camera vision of the ocean which you can compare to published sea and swell conditions.
https://www.mybeach.com.au/beach/cottesloe-beach/
The reports that I've seen say the car was found 35 m out to sea and 50 metres away (north) of groyne. No reports even mentioned any damage to the car or showed the picture until 12 years later. I never considered there may have been an accident and it was a former sluether that said they were told in PM's from another former sleuther (P*****) that we are not allowed to discuss, that there was an accident. They also suggested that the bag of JC's clothes was taken back the next morning to the mall where it seemed to get put in lost property, and left there on purpose. Whether true or not evidence on the car, not including the crush damage to roof, looks like there was an accident. All my opinion.
State Library of WA 1986 Cottesloe Beach.
Light poles with floodlights seen in background on groyne