Density is dependent upon temperature. If a sample of water is warmed, the water will expand and occupy a larger volume. However, the mass remains constant, so density decreases. Conversely, as the water is cooled, it contracts and the density increases. If the sample is diluted, the density decreases.
Specific gravity is the ratio of the densities of two substances.
For our purposes here, specific gravity is defined as the ratio of the density of a water sample to the density of some reference substance - in our case, pure water. A hydrometer measures specific gravity. In order to convert a hydrometer reading to salinity, you must consult the two tables published in Standard Methods . Now we come to the problem. This size of error is not insignificant if your data may be used for such purposes as determining saltwater intrusion, determining the effect of stormwater discharges on the salinity of estuaries, calculating salt budgets, or constructing circulation models.