Most seawater is like this. B. Distribution of water density 1. The oceans are somewhat stratified having warmer water on top and colder water at depth. Water density increases as temperature decreases, so the world's oceans tend to be stabley stratified with less dense water at the surface and denser water at depth. As seawater freezes, it excludes salts, thus sea ice is nearly fresh when melted.
Sea ice tends to float on denser salty water.
The freezing process also increase the salinity and thus density of the remaining polar water. This is one way to make very dense water that will sink and form bottom water for much of the world's oceans. In warm seas, especially the Mediterranean, evaporation produces quite dense warm waters. As a parcel of water's density changes, the water parcel will sink or rise until it reaches a water depth where other water has the same density. If we know the density distribution of the oceans in three dimensions, we can determine the circulation of the oceans.