Sadly once a keyboard gets water damaged there is little one can do to fix. Apple designed the keyboard & trackpad to be part of the uppercase assembly. The best way to fix this is to replace it. You'll need to find a junker with a good keyboard as presently I don't know of any one having it for sale other than getting it replaced by an Apple authorized service center if they are willing to do it. Liquid damaged systems are always an iffy situation.
For reference here is the 2015 models part you'll need. Sadly, it won't work in your system Apple altered the design in the 2016/17 models:
* [http://www.powerbookmedic.com/MacBook-12-Retina-Top-Case-with-Keyboard-Silver-p-84114.html|MacBook 12" Retina Top Case with Keyboard, Silver - Apple P/N 661-02242]
The fact the power button works is a good sign the rest of the system was spared. I assume the trackpad is also working as well.