This is a major cause of deep ocean currents. When ice freezes near the poles, the salt concentrates in the liquid water of the ocean. The remaining water becomes more salty and therefore more dense. The dense, salty water sinks deep into the ocean and creates a current. The density of water increases with depth.
Some animals change their depth by changing their density.
Recall that things that are more dense than their surroundings sink while those that are less dense than their surrounding float. Most fish have a swim bladder, a special sac that is filled with gases from their blood. When the fish's swim bladder is full, it is less dense than the surrounding water and the fish will rise towards the surface. Another property of water that affects lakes is the unique changes in density during phase changes. The density of most substances increases when a liquid becomes a solid.