Stratification

Sea water is in layers because of the different densities.
Surface Layer: down to about 200 m, most of the life is here
Intermediate Layer: 200-1000 m, changes in density, salt and temperature as the surface layer
Deep Layer: lower than 1000 m

Stratification is the layers of water according to its density there are two layers: there is warmer water overturning cold water, the water doesn't mix so pollutants usually stay in the same area.
Overturn is when changes in density causes water to mix (when polar water is cooled it sinks and the lower water comes up)