I would look at the boot.js file. It's kind of like arc.arc is for arc, in that it is a lisp-based bootstrap of all of the basic functions made available in the default environment.
It should give you basic examples of how to write code in wat, as well as show you which functions are defined. Of course, most of them don't actually have examples in the file, as it's merely defining the standard library, but it's a start. There's also test.wat, and the authors blog: http://axisofeval.blogspot.com/search/label/wat (which unfortunately mostly uses the underlying json representation for most of its code).
And the 'killer feature' of wat.js is not that it has first class environments or fexpr, which are admittedly very cool, but that the core language implementation that supports all of these and more is only a few hundred lines of code. There's more in the repository of course, but all of that is to provide the standard library or add sexpression parsing, etc. This makes wat a very good minimal platform for building a custom language.