It is easier for us to move through air than through water. This is because air is less dense than water.
The density of pure fresh water is 1-00 , while that of sea water is about 1.028 at atmospheric pressure and 0°C. Organisms living in water generally have a streamlined shape to help them move easily through water. Some organisms that float on the water surface are sensitive to changes in density. Eggs of aquatic organisms sink to different depths according to the density of the water.
