Covalent bonding happens when atoms share a pair of electrons. Covalent bonds are non-polar or polar. If the atoms share electrons equally, the bond is non-polar. If the atoms sharing is NOT equal, the bond is polar - slightly positive at one end and slightly negative at the other. Because of this, one water molecule attracts and hydrogen bonds with another.
Cells consist primarily of water and release a great deal of heat energy during metabolism.
Hydrogen bonds in liquid water keep cells from cooking. Temperature is a measure of the molecular motion of a given substance. In water, added energy disrupts hydrogen bonding between molecules, so there is NOT an increase in the motion of molecules. Hydrogen bonds can buffer large swings in temperature, helping stabilize the temperature of multi-celled organisms and of aquatic habitats. Additionally, hydrogen bonds in water are constantly breaking and reforming.