Gravity does not directly cause the rotation of a water molecule. Gravity is a force of attraction between objects with mass, and its primary effect is to attract objects towards each other. However, at the molecular level, gravity is relatively weak compared to other forces, such as electromagnetic forces.
The rotation of a water molecule, or any molecule for that matter, is primarily governed by electromagnetic interactions between its constituent atoms and their electrons. In the case of a water molecule (H2O), the two hydrogen atoms are bonded to the central oxygen atom through covalent bonds. These bonds create an electromagnetic force that holds the atoms together.
Within a water molecule, the oxygen atom's nucleus exerts an attractive force on the electrons shared between the oxygen and hydrogen atoms. At the same time, the oxygen atom's electrons also experience repulsive forces from the electrons of the hydrogen atoms. These interplay of attractive and repulsive forces gives rise to a stable molecular structure.
While gravity does not directly affect the rotation of a water molecule, it can influence the behavior of large bodies of water. For example, the gravitational force exerted by the Moon and the Sun causes the tides in Earth's oceans. These gravitational forces create a bulge in the water, causing it to move in a cyclical pattern. However, this effect is on a much larger scale than the rotation of individual water molecules.