I was looking at various websites looking for the best anti-freeze/coolant to use in a BMC "B" Series engine with an aluminium head. One site recommended such-and-such a product, and then said that the diluting water should not be hard tap water, as that could promote deposits to be produced and create hot-spots in the cooling system. That made sense. But then it went on to say that the correct water to use is de-ionised water, as is used in domestic irons. Through all my working life, de-ionised water (D.I.) has always had to be carried and stored in stainless steel, as it will eat plain iron and steel with a vengeance. To me, D.I. is the last possible stuff to put in your cooling system. Irons, yes, engines, no.
Which leaves the softened water from my house, which is softened so that it does not leave deposits in the pipework. Also it does not attack the pipework and eat it away. This is improved by the addition of system treatment, but in extremis is not essential. My area has Very Hard Water, and a house needs a softener.
What water do people usually use for engine coolant, or don't they care?