This is an awesome game, made as a project for the Udacity Frondend Nanodegree.
To begin you should clone, fork or download the project on a local folder.
You can run it by opening the index.html file on the project folder or setting up a simple server to run on the same folder and run localhost in your browser. One way of doing this is with Python 3 (you should have python pre installed) with the command python -m http.server [<portNo>].
Once you open the page with any of the previously mentioned options, the game will automatically start.
You should try to pass the enemies and get to the water line to increase your score. Game will get harder as you progress.
Enjoy it!