SocketIO is not compatible with C2's WebSocket plugin, unfortunately. Either you have to use a 3rd party plugin, or use a different Node.js library such as SockJS which I created a test project of a while back and it works perfectly.
https://github.com/sockjs/sockjs-node
This is my recommendation. There really aren't man WebSocket Node.js resources that have many recent update commits, or are even minutely usable by beginners. But I do like SockJS.