Water does its worst damage where you cannot see it. It gets into wiring looms, connectors and control modules, into the airbag system, the ABS unit, seat motors and the insulation under the carpet. A flooded car can look spotless after a deep clean and run fine for weeks — and then electrical faults, corrosion and mould start to appear months later.
Salt water is worse than fresh water: it corrodes metal and electronics much faster. But even a fresh-water flood that reached the dashboard usually makes a modern car a total loss for the insurer.