Just think a moment. The water molecule has its two hydrogens about 101 degrees apart, from the central oxygen.
Now why should it have anything magnetic? What could possibly create a north or south pole?
Sorry, microwave ovens work on the basis of the electric vector in electromagnetic waves. You can do the calculation yourself based on the electric dipole moment.
Many have a belief that magnetic bracelets and magnetic shoes are the key to health, and magnetism explains crop circles. I am not of this faith-based persuasion.
Because the atoms share electrons. The hydrogen atoms pick up a net negative electrical charge from the shared electrons they 'gain', and the oxygen atom picks up a net positive charge from the two electrons it 'looses'. Since the hydrogen molecules are only 101 degrees apart, their side of the water molecule has a net negative charge, and the other 'side' of the water molecule (the oxygen atom) has a net positive charge, hence the polar quality of the water molecules. The overall charge of the molecule remains neutral, but the charge is distributed to the polar ends.