While specific gravity is often based on the density of water, it is not a measure of water density itself. Specific gravity is simply the ratio between the densities of two different substances.
Although water is often used in specific gravity formulas for fluids, other substances can be used as points of reference. For example, specific gravity formulas for gases use air at room temperature as the reference. Furthermore, specific gravity does not use any units of measurement, because when two quantities with the same units are being divided into one another, the result is a pure number. It is therefore incorrect to say that specific gravity is a measurement.
The measurement of water's density is simply called density. The density of water varies based on temperature and is defined by the water's mass per unit of volume.