Anything that is more dense will result in being thicker and heavier. It will take more energy to move substances that have a higher density. Water is more dense than air. Water is normally in the liquid state of matter, meaning its molecules are fairly close together with some force of attraction for each other. Gas molecules, such as air, have no attraction for each other, and are therefore much less dense. The implications for this are obvious: since it requires more energy to move water over respiratory surfaces as opposed to air, the surface area of the respiratory organs must be increased. There should also be a higher flow rate of water passing over the respiratory surface than air passing over respiratory surfaces. Since energy requirements are higher, the mechanism that supplies the raw material, which would be oxygen, should deliver higher quantities. Since energy requirements are lower for air, that mechanism does not have to deliver as much quantity as does the mechanism for water.
♣ IT JUST THE TOPIC OF CHEMISTRY TOO....
☻ facts which let us spend more energy in water that in air are :-
☻ there is more spaces between the air molecules than of water so, thats why we need more force to overcome more spaces and force of attraction between water molecules.
☻ air molecules has very less or negligible force of attraction.
☻ air has less kinetic energy than water molecules which makes the molecules of air move with high speed.
☻ therefore, almost due to more space we are able to move through air easily than water.
FOR MORE SIMPLICITY..... AS WE KNOW THAT SOLID HAS MINIMUM INTRAMOLECULAR SPACES BETWEEN THEM ..THEREFORE WE CAN NOT MOVE OUR HAND THROUG THE SOLIDS