Volkswagon Beetle key help?

Well, the other day one of my friends accidently dropped my VW Bugs key in water. Of course its stopped working.. But my question is, do I need to get a whole new key? Or can I replace the battery in the old one? My car alarm automatically sets itself when the doors are locked, and I turned that off, but somehow recently it has turned itself back on. Scared the willies out of me when I tried to turn it on. And now I cant get it to deactivate again and I have to go through quite the process just to start my car. So.. Any help please??