The RAC Website gives this advice - What to do if you break down in the rain
If you break down in torrential rain, keep the bonnet closed while waiting for your RAC Breakdown patrol, to avoid the electrical system getting soaked.
RAC statistics show breakdown numbers increase significantly during periods of wet weather, as the damp causes problems with the engines and electrical systems, particularly in older vehicles.
If your engine cuts out after driving through deep water, do not attempt to restart it, as engine damage may occur – instead, turn on the hazard lights, call for assistance and have the vehicle professionally examined.
Catastrophic flood-related engine damage is typically caused by water being sucked into the engine, which causes the engine to lock up (mechanics call it an ‘air lock’) and can in turn damage important engine components such as piston connecting rods and valves.
This inevitably means a new engine will have to be fitted, but what people generally don’t understand is that it is the owner who is likely to have to foot the expensive garage bill, unless they can demonstrate to their insurer – like in any accident – that it was not their actions that caused the damage.