Ok, now I'm confused. I happened to run across another post on this forum here:
http://www.gotmead.com/forum/showthread.php?p=93143
It describes the "whys" of putting gypsum in the water. Gypsum is a salt that hardens water... thus it would seem that yeasties like hardness.
Of course it must be a matter of "how hard/soft is the source water?". I am sure there's a yeast mead "sweet-spot", in terms of the hardness values in ppm, TDS, or some other value. That led me to a Google search. I found this very informative powerpoint. It has some good comparative city water values for famous beer making cities. (It is beer-oriented.)
http://www.nthba.org/www/docs/Water_Treatment_April_2007_pp2003.ppt
Two good beer-oriented articles:
http://byo.com/water/item/1478-the-elements-of-brewing-water
http://www.winning-homebrew.com/brewing-water.html
And, of course, Wikipedia explains the general scale of what hard water is:
http://en.wikipedia.org/wiki/Hard_water#Hard.2Fsoft_classification
But, what's good for mead? Here's a post from a wine making board, it might give some ideas:
http://www.winemakingtalk.com/forum/f6/water-hardness-15769/#post147930
and, for what it's worth, here's a post about water hardness and mead:
http://forum.northernbrewer.com/viewtopic.php?f=8&t=114065
Also to consider is the natural mineral content of honey. See here for "standard nutrition facts" with mineral count:
http://www.benefits-of-honey.com/honey-nutrition.html
There is variation to consider. In the following report abstract, there is this important line: "
Variation in trace element content in different honey types is primarily
due to botanical origin rather than geographical and environmental exposition of nectar sources", with the keywords bolded.
http://www.agroscope.admin.ch/publikationen/einzelpublikation/index.html?lang=en&aid=10522&pid=12678
After this research, I'm thinking that the interplay of the specific honey you start with and the water are the most important things. Those are the two largest quantity ingredients in every mead. Hard water with a "hard honey" will have a different profile than hard water with a "soft honey", etc.
If you're really concerned about the water, I'm also starting to think it might be worth getting a test kit and doing a test of your tap water. Then, try 1 gal batches of short mead with the same honey, using bottled water, your tap water, or hardened water with gypsum. Make them more or less at the same time (to reduce temperature variations) with same yeast. See what flavors you like. There obviously isn't a right or a wrong answer. (If you are inclined to do this test, please let us know your findings!)
By the way, I use NYC tap water and so far the beer that I've made is really nice (no off flavors). I'm a new meadler, and have only one completed batch under my belt. It was a very nice show mead, comparable to a fresh, crisp and dry white wine. The batch that is currently fermenting also has nice flavors. I think my tap is fine to use. Though, they sometimes add extra chlorine to the water, and on those occasions, I might not make mead that day. NYC TDS average is 47. The water report, see page 10:
http://www.nyc.gov/html/dep/pdf/wsstate12.pdf