There's been a biblical amount of rain here in Houston over the last few weeks, and several of my friends have foolishly driven their cars into high water. So here's a list of things to do that I sent one person... who promptly ignored it all, tried to start the car, and broke a rod. Yet another chick that had to learn the hard way to listen to Ray. 
Did I miss anything?
- Rule of thumb: if water got over more than halfway up the dashboard, just have your insurance adjuster total the car.
- Disconnect the battery. Don’t do this when it’s wet, or if you’re standing in water.
- Check the oil. If it’s got water in it or is milky, drain and change the oil and the filter. Consider removing the oil pan for a complete cleaning. Do not attempt to start the car yet.
- Check the transmission fluid. If it’s got water in it or is milky, drain and change. Do not attempt to start the car yet.
- Visually inspect the computer to see if maybe it got soaked in water. If so, let it dry for a couple of days before attempting to start the car. Do not attempt to start the car yet. If you try starting the car without performing this step, you risk catastrophic damage to the computer due to electrical shorts.
- If water got in the fuel system through the fill hole, drain all fuel from the tank, and clean the tank of any debris. Purge the fuel lines and replace the fuel filter. Remove and clean the fuel rails and injectors. Reinstall everything.
- Check the air filter. If it’s dry, that’s a good sign that no water got in your engine through the air intake. If it’s wet, disconnect the entire air intake all the way to the throttle body. Inspect the inside of the air tubing for evidence of water. If none, that’s a good sign that no water got in your engine. If there is water present, then water probably got in your engine, which is bad. Replace the air filter with a new one, or let the old one dry out completely. If you try starting the car now, you risk catastrophic damage to the engine due to water being in the cylinders. Google hydro-lock.
- Remove the spark plugs and manually turn the engine over at least one full revolution. If you see water coming out of the spark plug holes, then you know that water did in fact get in your engine. Insert a vacuum line into each spark plug hole as far as it will go to vacuum the water out. To do a good, professional job, remove the heads and clean out each cylinder by hand, since the water is likely to be dirty and full of debris. Install new spark plugs or dry the old ones and re-install. Google hydro-lock.
- Reconnect everything you may have disconnected, and try starting the car. If the starter doesn't engage, it is probably wet, its terminals are wet, or the starter wires were disconnected by rapidly moving water. After making sure the battery is disconnected, check that the terminals are both connected and dry. If so, wait a couple of days to let it dry out, or remove it and dry it yourself.
- Change the oil and filter after a few hundred miles.
- Repack the wheel bearings and CV joints, if they're not sealed. This is not time-critical.
- If the car sat in water for more than a few hours, consider replacing all engine and transmission seals. This is not time-critical, but it’s a good idea to take care of this quickly. Water can get past those seals and destroy them after sitting in high water for a while; this will cause big oil leaks down the road.
Did I miss anything?
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