Answer to "Is it possible to replace JUST the keyboard on a MacBook Pro?"Yes you can replaced the keyboard on a Macbook Pro Unibody. In fact, I have successfully replaced several. However, it does require patience and the hands of a surgeon. I'm really not sure why some of you "Professionals" who suggest that it is impossible when all of you know that it is very possible to replace the keyboard on a Macbook Pro Unibody. Guess you guys need to sell top cases.
Answer to "Water Damage"You DO NOT have to disassemble your Macbook Pro. Just go to Radio Shack and purchase a can of NON CONDUCTIVE COMPONENT SPRAY and spray down the keyboard and everywhere that you can find an opening. Don't worry, remember it's NON CUNDUCTIVE Spray which means that electricity can not discharge so all of your internal components whould be safe. I recently had the same issue with my 2010 Macbook Pro i7 which you already know cost a small fortune. I used the spray and allowed the MBP to rest for 1 hour preferably in a warm location and that was it..Keyboard works, mouse pad works, usb works...Don't pay Apple $1200 for something you can do for your which cost only $12 per can.
Answer to "would you be able to fix water damage on a MacBook Pro?"Use NON Conductive Component spray which you could find at Radio Shack and generously spray down everything internal including the motherboard and all surrounding components. Allow the MBP to rest in a well ventilated area for 1 hour..it did the trick for my 2010 Macbook Pro i7 which I made add is working perfect.