If you have tested the water damaged iPhone with a new known working LCD and it is still not working then unfortunately your logic board has become a victim of liquid damage :(
You can try re-seating the connections if you haven't already, however we all know how much electrical components loooooooove liquid.
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My advice is to either return the iPhone to whoever you got it off, search for a replacement logic board on eBay or wait until the beautiful iPhone 5 is released.
If you have tested the water damaged iPhone with a new known working LCD and it is still not working then unfortunately your logic board has become a victim of liquid damage :(
You can try re-seating the connections if you haven't already, however we all know how much electrical components loooooooove liquid.