The density of pure water is 1000 kg/m3. Ocean water is more dense because of the salt in it. Density of ocean water at the sea surface is about 1027 kg/m3. There are two main factors that make ocean water more or less dense than about 1027 kg/m3. The first is the temperature of the water and the other us the salinity of the water. Ocean water gets more dense as temperature goes down. So, the colder the water, the more dense it is. Increasing salinity also increases the density of sea water. Less dense water floats on top of more dense water. Given two layers of water with the same salinity, the warmer water will float on top of the colder water. However, temperature has a greater effect on the density of water than does salinity. Hence, a layer of water (pycnocline) with higher salinity can actual float on top of water with lower salinity if the layer with higher salinity is quite a bit warmer than the lower salinity layer. The temperature of the ocean decreases significantly as you go to the bottom of the ocean. So, the density of ocean water increases somewhat proportionately as you go to or toward the bottom of the ocean. The deep ocean is layered with the most dense water on bottom and the lightest water on top. Circulation in the depths of the ocean is horizontal, a result of deviations in density in constrast to surface circulation which is a product of surface winds. That surface level movement is to some degree vertical and horizontal, perhaps swirling, but within the same general density layer. Hence, ocean water moves laterally along the layers with the same density. The density of ocean water is rarely measured directly. If you wanted to measure the density of ocean water but did not have the expensive equipment to do it directly, you would have to collect a sample of sea water and bring it back to a controlled environment to be measured. Density is usually calculated using a standard equation. You just need to measure the salinity, temperature and pressure to be able to find density.
We thank the University of Michigan and the NOAA for assistance with this calculator and information.