Car was only in water a matter of hours, forensic tests done on battery, light on. It was found under a light post, by early morning beach goers. Cott beach even in winter has swimmers every morning, runners, fisherman, cars going past on Marine Parade, paid beach ranger / lifeguard (or whatever name was then).
Back seat was found two days earlier (Monday afternoon), just on south side of groyne by beach walkers. This indicates to me, it was removed and thrown off groyne on the night or early morning JC went missing. It ended up not far south of groyne with prevalent waves and drift. Even in the stormy weather conditions your're suggesting on the Monday 20th, in winter people were walking along the beach and found the back seat washed up on sand.
IMO the car went in the ocean just a couple of hours before it was found early Wednesday morning, on the Tuesday night. With the damage to the car, which I think occurred away from the ocean, it could have easily been disposed of anywhere else relatively easy, but it ended up at Cott, extremely hard to explain!