I don't think there's any likely about it, submerged cars are considerably heavier with the additional silt/sand that gets into everywhere. It gets into the carpet/seat cloth weave, the headlining cavity, the ventilation system, anyway into the engine, anyway past bearing seals, into steering rack gaiters, into electrical systems, literally everywhere!. Any bodywork damage is irrelevant.
I don't know about a vehicle be written off, because it was submerged in sea water.
When I lived at the Aquatic Gap in Barbados, we had a tropical storm that lasted a number of days, with the end results that there was major flooding. I had a British Leyland 1100 at the time, and this vehicle was eventually submerged. When the floods went down, we were expecting the worse.
It didn't turn out like that, because after we had changed the engine oil and filters, changed the fuel pump and cleaned out the silt from the interior, the vehicle was once again drivable. It did take a little longer though to remove the smell from the car interior, strong disinfectant daily and wide open windows was a necessity for quite a few days after.
Another incident while I was living and working there, was a person who we knew, and who had been trying hard to sell his car. On travelling by the sea port we saw a crowd gathering, so made an investigation. Lo and behold there was our now wet car seller who stated "Been an drown me car man".
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