Pycnocline

So apparently the water density of the ocean is about 1027kg/m3. Two main factors affect water density : temperature and ocean salinity. As temperature decreases density increases, which makes sense because usually colder things are more dense.
The fact that there is difference in density means that less dense water floats on top of more dense water...pretty straight forward stuff. However, temperature has a greater effect on water density than salinity so that a more salty layer of water can actually float on top of a less salty layer...which is kinda amazing phenomenon. Temperature of the ocean water decreases with depth so there is a horizontal temperature gradient to ocean water. A pycnocline is a rapid decrease of density with depth. The image shows a picnocline which I got from here.