What happens to a car after it has been underwater?
Water damage is not one problem but several, and some appear weeks later. Treat the engine, the fluids, the electrics, the chassis and the interior separately. The first rule: do not try to start the engine.
Engine
If water reached the air intake, an engine that is cranked can draw water into the cylinders. Water does not compress: the piston stops on it, and connecting rods can bend or break (hydrolock). Before any start attempt, the air filter and intake are checked, and spark plugs or injectors removed to turn the engine by hand and expel water. Milky oil on the dipstick means water in the sump: oil and filter must be changed, sometimes more than once.
Fluids and drivetrain
Gearboxes, transfer cases and differentials breathe through vents. Submerged, they can take in water. Their oils are checked and changed if contaminated, along with brake fluid if water may have entered the reservoir. Wheel bearings and CV joints can also take in water and corrode.
Electrics
Many control units and connectors sit low: under seats, in footwells, in the boot floor — including airbag and body control modules on many cars. Water and especially salty or dirty water corrode connectors and circuit boards. Faults can appear days or weeks later as corrosion progresses. The battery is disconnected immediately, connectors are opened, dried and inspected, and modules assessed.
Chassis and body cavities
Sills, chassis rails, doors and cavities have drain holes designed to let water out. Mud and debris block them, keeping water inside and starting corrosion from within. The drains are cleared, cavities flushed and dried, and treated with cavity wax where needed. Doors are drained and checked.
Interior
Carpets sit on thick acoustic underlay foam that holds water like a sponge. Surface drying is not enough: carpets and underlay are removed, dried or replaced, and the floor pan dried, otherwise mould and smell follow. Seat foam behaves the same way. Wiring under the carpets is inspected at the same time.
What to do first
Do not start the engine. Disconnect the battery if safe to do so. Have the car moved by recovery, not driven. Note how high the water reached — it decides which systems must be inspected.