Warming water makes it lighter. Dissolving salt in it makes it heavier. So then, which of hot saltwater and cold freshwater is heavier? The density of sea water is determined by its temperature and salinity. Warm sea water is not necessarily lighter than cold sea water. Around the equator the water temperature is around 30 degrees Celsius, and near the North and South Poles, it is at the ice-forming temperature of minus two degrees. However, the concentration of salt at the equator is higher than that of the poles, so the density of water does not vary much, and the water at the poles is actually lighter in some cases.