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.
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.