I drove the Mirai a few years ago at a Toyota press event and one of the questions I had for the technical chaps was to enquire about the H2O button on the dash. They said it is to release the stored water generated as a by-product of the hydrogen conversion.
Apparently, it is advisable to release the water prior to parking in your garage. I then also asked whether it was pure water or any other contaminants. They replied it was indeed pure water and although you could probably drink it maybe it wasn't the best source to get a drink. However, if it converted to Beer, then I am definitely putting my name down for a Mirai 🙂