Here is what I was taught about cars and flood damage when I was young and impressionable:

• Thou Shalt Not Purchase A Vehicle With A Salvage or Flood Title. Corollary: When Purchasing A Used Vehicle, Thou Shalt Watch Carefully For Title Washing Fraud.
• Thou Shalt Insist That Thy Flooded Vehicle Be Declared A Total Loss.

I've been fortunate enough to never have a vehicle get flooded (although one time the water was over the hubs and getting pretty close to the exhaust before I was able to move it). I'd be interested to hear if someone knows better than I was taught.