The earliest reference I could find (not including that Brooke reference, which I think is erroneous) is 1712 in the The Natural History of Northamptonshire by John Morton. He says:

The common springs that have a little, slight Tincture of Vitriol, which with us are usually called Hard and harsh Waters, are vulgarly known to be such by their not taking so speedy or so lasting a Lather with Soap; by their turning and curdling of Milk in case a smaller Quantity of Milk is infus'd upon a much larger Quantity of the Boyling Water; by their requiring a larger Quantity of Malt to make the Drink, that's brew'd with it, of equal Strength to that which is made with softer Waters; and by their not boyling Pease, and other Garden Stuff so quickly or so soft and tender, as do those of a milder Nature. A Person of a nice Palate may distinguish them by the Tast; And to one who washes with them, whose Skin is of a finer Texture, they are sensibly harsh, and leave a little Roughness on the Hands and Face.

Harsh water actually seems to be an earlier term for the same thing, being found as early as 1702 in The History of the Royal-Society of London, for the Improving of Natural Knowledge by Thomas Sprat (earlier if you count the l'eau aspre) and being the opposite of soft water. Perhaps hard water was derived from soft water as being more clearly its opposite.

The earliest reference I could find (not including that Brooke reference, which I think is erroneous) is 1712 in the The Natural History of Northamptonshire by John Morton. He says:

The common springs that have a little, slight Tincture of Vitriol, which with us are usually called Hard and harsh Waters, are vulgarly known to be such by their not taking so speedy or so lasting a Lather with Soap; by their turning and curdling of Milk in case a smaller Quantity of Milk is infus'd upon a much larger Quantity of the Boyling Water; by their requiring a larger Quantity of Malt to make the Drink, that's brew'd with it, of equal Strength to that which is made with softer Waters; and by their not boyling Pease, and other Garden Stuff so quickly or so soft and tender, as do those of a milder Nature. A Person of a nice Palate may distinguish them by the Tast; And to one who washes with them, whose Skin is of a finer Texture, they are sensibly harsh, and leave a little Roughness on the Hands and Face.

Harsh water actually seems to be an earlier term for the same thing, being found as early as 1667 in The History of the Royal-Society of London, for the Improving of Natural Knowledge by Thomas Sprat (earlier if you count the l'eau aspre) and being the opposite of soft water. Perhaps hard water was derived from soft water as being more clearly its opposite.