Good question. A metal detector?
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Easy. Sonarscanner. The company I work for it doing business in off-shore oil serveying. A couple of months back they found the remains of a French dreadnought era battleship. by chance.[[Check out the video!) I was not involved in this. I'm a landlubber who's doing something totally different. There's a 1:3 scale model of that orange fish in the lobby of the head office. And I'm sure the servey ship itself is also somewhere as a scale model. We've got loads of them, also in the lobby.
http://upload.wikimedia.org/wikipedi...r-img_3137.jpg
The French battleship Danton in better days.
Searching for cars in a river is a walk in the park.
Plus wasn't the river wider at once time? Who knows what kind of stuff is still down there. I am sure there are a few destroyed rumrunners w/ cargo as well as artillery pieces.
Now you're talkin'. What we really need is a webcam on a sub, both remotely controllable over the internet like the Dossincam. How cool would that be? We could all collaborate searching for interesting artifacts down there, SETI-like.