The coffee is two percent of what ends up in the mug. The rest comes out of the tap, and for a long time I did not think about it at all.
Our water here is hard. Not dramatically so, but enough that the kettle grows a crust every few months and that every light roast I buy tastes flat and slightly chalky. I assumed for a year that I was grinding badly.
The test that convinced me
I brewed the same coffee three times in one morning: tap water, bottled water with low mineral content, and a half-and-half mix. Same dose, same grind, same pour. The tap cup was dull, the bottled cup was bright and a little sour, and the mix was, annoyingly, the best of the three.
Minerals carry flavour out of the grounds, so water with almost nothing in it extracts unevenly and tastes hollow. Water with too much of the wrong kind buffers the acidity away and leaves everything tasting the same. The middle is where the coffee actually shows up.
What I do now
I keep a five litre bottle of low mineral water and cut it with tap water, roughly one to one, adjusting by taste when I change beans. No filters, no sachets, no scales beyond the one I already own. The difference is larger than any grinder upgrade I have made.
If you only change one thing this month, change the water. It is the cheapest experiment in the kitchen and the only one where the result is obvious in the first sip.