A compromised electrical system is a critical and potentially life-threatening hazard inherent in cars exposed to high waters for extended periods of time. You'll want to test the car extensively to ensure all electrical components are operational.
First, poke your head under the dash and gently bend the electrical wires to see if they're brittle. If they are, you've likely detected water damage. Water damage to the rest of the electrical system is harder to view, however, so you'll need to take the car on a test drive and try out all the electronics.
When you turn the ignition, listen for unexpected sounds, and use your eyes and nose to see if smoke appears anywhere. Be sure all the dashboard lights come on, including the back lighting, and check headlights, turn signals and emergency blinkers.
Turn on the air conditioning, wipers and cigarette lighter to be sure they work as expected. And don't forget to listen to the radio: Static-plagued or distorted audio, or no audio at all, could be the result of water damage.