Explanation:

Salinity decreases but density of sea increases with depth because:

• The salinity decreases from the surface ocean to deep waters is very small, from about 36 g/L (ppt) at the surface to 35 g/L (ppt) in the deep water, thus there is a very small density decrease with depth given a constant temperature.
• The density of seawater can be increased by reducing its temperature, increasing its salinity, or increasing the pressure. Pressure has the least impact on density as water is fairly incompressible, so pressure effects are not very significant except at extreme depths. 
• As depth increases, there is a region of rapidly increasing density with increasing depth, which is called the pycnocline. The pycnocline coincides with the thermocline, as it is the sudden decrease in temperature that leads to the increase in density. By creating a stratified water column, the thermocline and pycnocline together create a barrier that prevents mixing between the warmer, less dense surface water and the colder, denser bottom water. In this way, nutrient-rich deep water may be prevented from coming to the surface to support primary production.