After a little digging I learned the method originally comes from I'm just popping in to tell you I finally tried it, and it worked. You should try it too. Because clearly, since we're making mousse with water instead of heavy cream and butter, we can call it healthy and justify eating one silky spoonful after another.
There was some sort of weird magnetic attraction between my finger and my tongue throughout this process. It sort of interfered with my ability to give you specific measurements. Accounting for this variable, which you too will likely encounter, I'd say start with 5 ounces of 72% dark chocolate and 1/2 cup water. Sample and adjust accordingly. That is, assuming you have any left to adjust.