When swimming in salt water you can make observations on more than just the salty taste. An observation you may have made is that you are more buoyant, or you float more, in salt water than in fresh water. This is a result of the oceans salinity, the amount of salt in the water. Density is the amount of matter in a given space or area. When a volume of water is replaced by an item of greater density than the water, the object will sink. Likewise, if a volume of water is replaced with an object that is less dense than the water, it will float towards the surface.
This physical happening can be observed when pouring fresh water into a pitcher of salt water. The fresh water, when poured slowly, will remain on the top of the salt water. A small layer forms where the two bodies of water meet, called a Halocline. You can also observe the difference in density by using an egg. Place an egg in fresh water and watch as it sinks to the bottom. When placed in salt water it will float. The egg was denser than the space it replaced in the fresh water causing it to sink and was less dense than the salt water causing it to float. When placing the egg into the pitcher that had the fresh and salt water the egg floated right at the Halocline.