Water density is a measure of the volume weight of water. It is important for the proper calculation of displacement. Water density is represented by the Greek letter rho (ρ). Water density varies principally with salinity (higher density with more salinity), and slightly with temperature (maximum density about 4c, this is a non-linear relationship). Because it can vary environmentally, the values represented here are typical rather than prescriptive - for accurate values, hydrometers are used to measure water density (i.e., when performing inclining tests).
Relative density or specific gravity is the ratio of the density of one cubic meter of a given substance to one cubic meter of freshwater (1000kg). This value is a unitless ratio.