My friend, if you are expecting us to provide you some harry potter like magic to fix your water affected keyboard, my advice would be to bang your head right onto it (the keyboard).
On a serious note, buy a new keyboard. Get once, get genuine. Spend some money and move your @$$ to the mkt. If buying from outside of Pk is an option, get an original one from the manufacturer. I reckon there would be any warranty remaining. Besides I doubt water damage is covered in warranty. Get a manufacturer replacement and replace your keyboard. Replacing it is very easy, unless you're a noob at hardware and mechanics.
EDIT: Wait a min, the down key wasnt working before and is working after the water damage? :O You sure it isn't a problem of the keyboard wire panel not seated properly? Could be possible. I dont think the water got in to damage the keyboard. If that was the case it would be functioning stupendously with some keys producing weird results and others not working. The most common is the asdf and ghjkl set failure. But open up your lappy to see if its weird connector is slightly out of place. Try resetting it even if you think its good. Oh and dont lose the screws. Use a processor box or a pan to keep em safe.