https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3775162/Pallav Sengupta, 2013 Aug; 4
In the past five decades or so evidence has been accumulating about an environmental factor, which appears to be influencing mortality, in particular, cardiovascular mortality, and this is the hardness of the drinking water. ...
Hard water has no known adverse health effect, WHO says at its Geneva Conference. In addition, hard water, particularly very hard water, could provide an important supplementary contribution to total calcium and magnesium intake. ...
In most large-scale studies, an inverse relationship between the hardness of drinking-water and cardiovascular disease has been reported. ...
Important findings in this field were provided recently by Taiwanese scientists. In most of their studies, the authors indicated a negative statistical association of various types of cancer morbidity/mortality with the hardness of water and calcium. ...
For years, I've been (jokingly) telling people that I drink tap water because "calcium and iron are good for you". Apparently, there are studies that back this up.
Read the full article for all the details. The upshot of it is that although additional study is needed, it seems that hard water is either beneficial to health or has no impact, depending the health aspect in question.