Why do you use water to determine specific gravity?

Water was used as the standard because at first, the cubic centimetre was defined so that pure water, at 4 deg C and a pressure of 1 bar, had a density of 1 gram per cubic centimetre. The specific gravity of other substances, being the ratio of their density to that of water, meant that their specific gravity would have the same numeric value as their density. However, an error in defining the centimetre resulted in a change in its definition and the density of water is no longer 1 g/cm3.