What is the proper way to write the density of water?
As one cubic centimeter is one mL, the two are mathematically identical. In S.I., densities are in kg per cubic meter; in cgs, it is in g per cubic centimeter. Density of water is 1000 kg/m^3. SI is preferred in scientific publications. The number of significant figures depends upon the application — you have four sig. figs in the first density; one in the second. The actual density of pure water at 4°C is 0.9999720 so both your quotes are “approximations”. The definition of the kg is based on something else.
