The negative charge in a water molecule occurs because

The unequal sharing of electrons gives the water molecule a slight negative charge near its oxygen atom and a slight positive charge near its hydrogen atoms. When a neutral molecule has a positive area at one end and a negative area at the other, it is a polar molecule.

Explanation:

the relative standard reduction potential of the half-cell in which reduction occurs; more positive than the other half-cell. reduction is a chemical process in which electrons are added to an atom or an ion; it always occurs accompanied by oxidation of the reducing agent.