Put an egg in water and observ that it does not float.
Put salt in the water and see the egg start to float on the water. Mix more water in, and see it sink again.
This is an easy experiment to do in the kitchen, but such a great part of how density works.
When we swim in the ocean we will find it easier to float or swim, than in freshwater. This is because the water density is higher in saltwater, than in freshwater.
If we would try to swim in oil, which has a lower density than freshwater and saltwater, we would find it very hard to stay above the surface.