Honestly, you simply want some alkalinity in the water to help buffer acid produced by the yeast and the acidity of the honey. Beyond that, it's preference.
I don't care much for hard water in mine, so when I use RO, I just add 8 grams of gypsum and calcium chloride, and about 5 grams of potassium bicarbonate for a buffer. That's for 5 gallons total batch size.
Of late, I have gone back to cheap spring water from the grocery. It's convenient, pre measured, and I can reuse the container.
Many of the mead masters will just use good tap water treated with campden tabs. Of course, most have access to moderate alkaline waters (120-160 ppm alkalinity).
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